Network data transmission synchronization system and method

ABSTRACT

A number of features for enhancing the performance of a cable transmission system in which data is transmitted between a cable modem termination system at a headend and a plurality of cable modems located different distances from the headend. The power transmission level, slot timing, and equalization of the cable modems are set by a ranging process. Data is transmitted by the modems in fragmented form. Various measures are taken to make transmission from the cable modems robust. The upstream data transmission is controlled to permit multiple access from the cable modems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/574,558, filed on May 19, 2000, which is a continuation-in-part ofU.S. regular application Ser. No. 09/430,821, filed on Oct. 29, 1999,which claimed the benefit of the filing date of U.S. Provisional PatentApplication No. 60/106,264, filed Oct. 30, 1998 and entitled HEADENDUPSTREAM MAC/PHY INTERFACE; U.S. Provisional Patent Application No.60/106,427, filed Oct. 30, 1998 and entitled ROBUST TECHNIQUE FOROPTIMAL UPSTREAM COMMUNICATION BETWEEN CABLE MODEM SUBSCRIBER AND AHEADEND; U.S. Provisional Patent Application No. 60/106,438, filed Oct.30, 1998 and entitled SYSTEM FOR, AND METHOD OF, FRAGMENTING DATAPACKETS IN A CABLE MODEM SYSTEM; U.S. Provisional Patent Application No.60/106,439, filed Oct. 30, 1998 and entitled CABLE MODEM SYSTEM; U.S.Provisional Patent Application No. 60/106,440, filed Oct. 30, 1998 andentitled NETWORK DATA TRANSMISSION SYNCHRONIZATION SYSTEM AND METHOD;U.S. Provisional Patent Application No. 60/106,441, filed Oct. 30, 1998and entitled BURST RECEIVER SYSTEM, the entire contents of all of whichare hereby expressly incorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to communication systems. Thepresent invention relates more particularly to a cable modem systemwherein information is communicated between a plurality of cable modemsand a cable modem termination system.

BACKGROUND OF THE INVENTION

The desired solution for high speed data communications appears to becable modem. Cable modem is capable of providing data rates as high as56 Mbps, and is thus suitable for high speed file transfer, videoteleconferencing and pay-per-view television. Further, cable modems maysimultaneously provide high speed Internet access, digital television(such as pay-per-view) and digital telephony.

Although cable modems are used in a shared access system, wherein aplurality of subscribers compete for bandwidth over a common coaxialcable, any undesirable reduction in actual data rate is easilycontrolled simply by limiting the number of shared users on each system.In this manner, each user is assured of a sufficient data rate toprovide uninterrupted video teleconferencing or pay-per-view television,for example.

SUMMARY OF THE INVENTION

The present invention specifically addresses and alleviates theabove-mentioned deficiencies associated with the prior art.

According to an aspect of the invention a modem communicates with acable transmission system. The modem requests an amount of bandwidth onthe cable system to transmit data. The modem receives a grant of anamount of bandwidth to transmit data in response to the request. Themodem compares the requested amount of bandwidth with the granted amountof bandwidth. Responsive to the comparison, the modem fragments the datato be transmitted into a plurality of segments if the requested amountis larger than the granted amount. One of the segments is no larger thanthe granted amount. Responsive to the grant, the modem transmits all thedata to be transmitted to the cable system if the requested amount is nolarger than the granted amount and transmits only the one segment to thecable system if the requested amount is larger than the granted amount.

In another aspect, the present invention includes a method forfacilitating communications between a plurality of cable modems and acable modem termination system. The method comprises transmitting arequest from the cable modem to the cable modem termination system. Therequest is a request to transmit a data packet from the cable modem tothe cable modem termination system. A time slot for transmitting thedata packet from the cable modem to the cable modem termination systemis allocated, optionally by the cable modem termination system.Information representative of the time slot is then transmitted,optionally by the cable modem termination system, to the cable modem.The cable modem then transmits the data packet from the cable modem tothe cable modem termination system within the allocated time slot.

Transmitting the data packet from the cable modem to the cable modemtermination system within the time slot mitigates collisions betweendata packets which are transmitted by different cable modems to thecable modem termination system at the same time and upon the samefrequency channel.

In another aspect, the present invention includes an apparatus andmethod for synchronizing upstream communications between a plurality ofcable modems and a cable modem termination system (CMTS). A firstmessage representative of a frequency of a clock of the cable modemtermination system is generated, particularly by the cable modemtermination system. Similarly, a second message representative of a slottiming offset of a selected one of the cable modems with respect to theclock of the cable modem termination system is generated, againparticularly by the cable modem termination system. The first and secondmessages are transmitted to the selected cable modem. Frequencysynchronization of the clock of the selected cable modem is enhancedwith respect to the cable modem termination system utilizing the firstmessage. The slot timing offset of the clock of the cable modem withrespect to the clock of the cable modem termination system iscompensated for using the second message. This process is repeated, asnecessary, for each cable modem which is to communicate with the cablemodem termination system.

In another aspect, the present invention includes an apparatus andmethod for rapidly acquiring data packets in a receiver such as that ofa cable modem termination system. The method generally comprisesdetermining fractional symbol timing correction by a feedback loopprocess, determining carrier phase correction by a feedback loop processand determining fine amplitude by a conventional coherent estimationprocess.

Optionally, the amplitude of the input to the fractional symbol timingcorrection circuit and/or the input gain to the carrier phase correctioncircuit may be adjusted so as to be within the optimal operational rangeof the fractional symbol timing correction circuit and/or the carrierphase correction circuit via the use of phase detector gain boostinglogic, wherein a sensor senses the amplitude of the signal input to thefractional symbol timing correction circuit and/or the carrier phasecorrection circuit and the amplitude of the signal is then modified soas to be within the optimal range.

Optionally, timing recovery may be accelerated by utilizing twoout-of-phase, e.g., 180 degree out-of-phase, sampling timing signalssuch that one of the two sampling timing signals will always be locatedsufficiently distant from a null point in a binary input sequence as toenhance timing recovery.

Another aspect of the present invention is a method for communicatingmodulated information from a plurality of cable modems to a cable modemtermination system in a way that enhances the robustness of the upstreamchannels. One way is to monitor upstream channels for at least onemodulation parameter which is indicative of channel quality andadjusting the modulation in response to the channel quality parameter.

Another way is to vary the symbol rate of information transmitted over achannel in a near-continuous manner by varying at least one of the upperand lower bounds.

Another way is to monitor the upstream channels and to use fine carrierfrequency agility and/or variable symbol rate to facilitate dynamicchannel allocation so as to communicate only within channels havingquality which is above a predetermined threshold.

According to another aspect of the present invention, slot timinginformation and/or data-type information is transmitted from a MAC to aburst receiver so as to facilitate processing of upstream data packetsby the burst receiver in an efficient and reliable manner. Channelstatus/error information is transmitted from the burst receiver to theMAC. The error information is used by the MAC to facilitate spectrummanagement which enhances the data rate and/or reliability of upstreamcommunications.

In an aspect of the invention, noise is abated in a cable system. Thecable system has a headend and a plurality of user terminalsinterconnected by a cable that has a single downstream transmission pathfrom the headend to each of the user terminals and a plurality ofupstream transmission paths from each of the user terminals to theheadend. A notch filter at the headend is adjusted to reject one or morebands of common noise. A feedback equalizer (FBE) at the headendcompensates for the distortion caused by the notch filter. Afterwards, aranging signal is received at the headend from each user terminal andfeedforward equalizer coefficients corresponding to the transmitequalizer coefficients for each user terminal are generated from theranging signals. The feedforward equalizer (FFE) coefficients aretransmitted to the respective user terminals, where they are applied toa transmit equalizer filter to compensate for individual noise at therespective user terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will be more fully understood when considered with respect tothe following detailed description, appended claims and accompanyingdrawings wherein:

FIG. 1 is a schematic diagram of a hybrid fiber coaxial (HFC) networkshowing typical pathways for data transmission between the headend(which contains the cable modem termination system) and a plurality ofhomes (each of which contain a cable modem);

FIG. 2 is a simplified block diagram of a cable modem system wherein aline card which defines a cable modem termination system CMTS) isdisposed at the headend and a cable modem is disposed within arepresentative home;

FIG. 3 is a simplified block diagram showing the use of a fractionalsymbol timing loop, a carrier phase correction loop and a conventionalamplitude estimator to enhance the rate at which acquisition of datapackets is performed in a burst receiver of a cable modem terminationsystem or the like;

FIG. 4 is a block diagram showing the interrelationships of the bursttransmitter, subscriber medium access control (MAC) and receiver of thecable modem with the burst receiver, medium access control (MAC) andtransmitter of the cable modem termination system;

FIG. 5A is a schematics block diagram showing the interconnections ofthe burst receiver, medium access control (MAC) and transmitterdownstream modulator within a cable modem termination system;

FIG. 5B is a schematic block diagram showing the construction of thecable modem, shown in FIG. 2, at the subscriber, such as the home;

FIG. 6A is a block diagram showing a cable modem termination system anda representative cable modem communicating with one another via a cableplant;

FIG. 6B is a block diagram showing the cable modem termination systemand cable modem of FIG. 2 in further detail;

FIG. 6C is a block diagram showing the cable modem termination system ofFIG. 2 in further detail;

FIG. 6D is a block diagram showing the cable modem of FIG. 3 in furtherdetail;

FIG. 6E is a table showing an example of loop filter coarse coefficientsand fine coefficients which provide specified bandwidths at the listedupdate rates;

FIGS. 7A and 7B are block diagrams of a sub-system at the subscribermodem for receiving packets with encrypted data and control information,parsing the encrypted data from the control information, decrypting theencrypted data and separately storing the decrypted data and the controlinformation and for restoring the packets with the encrypted data andthe control information at the subscriber modem for transmission to theheadend;

FIGS. 8A and 8B are block diagrams of a sub-system similar to that shownin FIGS. 7A and 7B (but at the headend) for providing a parsing of thesignal packets received from the subscriber modem and a decryption ofthe encrypted data parsed from the packets and for providing anencryption of data for transmission to the subscriber modem and areformulation of the packets from the encrypted data and the controlinformation;

FIG. 9 is a block diagram in some additional detail of a burst receivershown as a single block in FIG. 4;

FIG. 10 is a block diagram in significantly increased detail of theburst receiver shown as a single block in FIG. 4;

FIG. 11 is a schematic diagram illustrating the round trip transmissiondelay between a headend and a subscriber modem;

FIG. 12 is a flowchart showing the software level synchronizationcontrol of a cable modem;

FIG. 13 is a flowchart showing the hardware level synchronizationcontrol of a cable modem;

FIG. 14 shows a continuous data stream, such as that which may bereceived by a conventional continuous receiver;

FIG. 15 shows a plurality of data bursts separated by guard bands, suchas those transmitted by cable modems to a cable modem termination systemaccording to time division multiple access (TDMA);

FIG. 16 shows in further detail an exemplary data burst of FIG. 15;

FIG. 17 shows the QPSK preamble of FIG. 16 in further detail;

FIG. 18 is a block diagram of a contemporary phase locked loop;

FIG. 19 is a block diagram of a fractional symbol timing loop in atypical digital receiver, wherein the matched filter is within the loop;

FIG. 20 is a block diagram of the fractional symbol timing loop of thepresent invention, wherein the matched filter has been moved outside thefractional symbol timing loop;

FIG. 21 is a block diagram showing a burst receiver having a fractionalsymbol timing loop, a carrier phase correction loop and an amplitudeestimator so as to effect fast acquisition of data packets;

FIG. 22 is a block diagram showing a burst receiver having a fractionalsymbol timing loop, a carrier phase correction loop and an amplitudeestimator, wherein the matched filter has been moved outside of thefractional symbol timing loop;

FIG. 23 is a block diagram of a phase detector gain boosting logiccircuit wherein the amplitude of a signal input to a phase detector ismonitored by a sensor and the amplitude of the signal to the low passfilter of the loop is controlled by the output of the sensor;

FIG. 24 is a timing diagram showing the use of a single contemporaryclock signal to provide timing for a sampling circuit contemporary clocksignal (FIG. 24-A) to provide timing for a sampling circuit and alsoshowing the use of two out-of-phase clock signals (FIG. 24-B), whereinand also showing the use of two out-of-phase clock signals, wherein oneof the two out-of-phase clock signals will always have a timingrelationship relative to the input binary signal to effect sampling ofthe input binary signal.

FIG. 25A is a schematic block diagram of a system for allocatingdifferent portions of a dynamic range of power between analog anddigital states in the system.

FIG. 25B is a schematic block diagram of an RMS estimator that is usedto derive a variable gain amplifier setting.

FIG. 26 is a block diagram of a prior art technique showing a pluralityof contemporary demodulators coupled to demodulate data which is inputfrom a transmission medium such as a fiber optic or coaxial cable andwhich is coupled to provide the demodulated data as an output thereof;

FIG. 27 is a block diagram of one aspect of the present invention,showing a monitoring circuit coupled to monitor a plurality of upstreamchannels for at least one parameter which is indicative of channelquality;

FIG. 28 is a block diagram of a prior art upstream burst receiver andmedium access control (MAC) showing modulated data input from atransmission medium, such as a coaxial cable, to the upstream burstreceiver and showing digital data output from the MAC;

FIG. 29 is a block diagram showing an aspect of the present invention;

FIG. 30 is a chart showing RS coding gain for various T's using 16-QAMwith K equals 64 bytes;

FIG. 31 is a schematic drawing providing an example of fine frequencyagility, wherein the frequency spectrum is divided into a plurality ofclosely spaced channels;

FIG. 32 is a flowchart showing dynamic channel allocation control flow;

FIG. 33 is a flowchart showing CMTS dynamic channel allocation controlflow;

FIG. 34 is a simplified block diagram showing the MAC/PHY interface ofthe present invention;

FIG. 35 is a schematic representation of a data packet showing thepositioning of the data or payload therein and also showing the locationof a guard band;

FIG. 36 is a schematic diagram showing the formation of an exemplary MAPwhich is transmitted by the cable modem termination system (CMTS) to allof the cable modems on a particular channel so as to facilitatecommunication of the cable modems with the cable modem terminationsystem according to a time division multiple access (TDMA) protocolwhich avoids collisions among data packets from different cable modems;

FIG. 37 is a schematic diagram showing the formation of frames by acable modem in response to receipt of a MAP, such as that shown in FIG.36;

FIG. 38 is a flowchart showing the operation of the cable modemtermination system in separating high priority requests and low priorityrequests received from cable modems;

FIGS. 39 and 40, taken together, define a flowchart showing theoperation of the cable modem termination system in granting requestsfrom cable modems to transmit data from the cable modems to the cablemodem termination system;

FIGS. 41 and 42, taken together, define a block diagram of that portionof the cable modem termination system which receives requests from thecable modems and which generates MAPs in response to these requests andalso shows a plurality of cable modems which receive, the MAPs and whichgenerate frames in accordance with the MAPs;

FIG. 43 is a graphical representation of the relationship of theminislots which define the request interval, maintenance interval anddata interval with respect to the minislot clock (MSCLK);

FIG. 44 is a graphical representation of the MAP message format prior tomessage filtering;

FIG. 45 is a graphical representation of the MAP message format aftermessage filtering;

FIG. 46 is a block diagram showing the architecture of the sharedSRAM-based MAC interface for eight upstream channels;

FIG. 47 is a block diagram showing the MAP timing control interfacesignals which are transmitted from the MAP to the demodulator of theburst receiver;

FIG. 48 is a graphical representation of the relationship between theminislots which define the request interval, the maintenance intervaland the data interval with respect to the minislot clock, MapValidsignal and MapData and also showing the MAP clock;

FIG. 49 is a graphical representation of the relationship between theminislots which define the maintenance interval, the minislot clock, theMapValid signal and MapData and also showing the timing of the receivenow (Rx now) signal;

FIG. 50 is a graphical representation of the relationship between theminislots which define the data interval, the minislot clock, theMapValid signal and MapData and also showing the timing of the receivenow (Rx now) signal;

FIG. 51 is a graphical representation of the relationship between theminislots which define the request interval, the minislot clock, theMapValid signal and MapData and also showing the timing of the receivenow (Rx now) signals;

FIG. 52 is a graphical representation showing the prepended informationwhen the first block TDMA transmission bit is set;

FIG. 53 is a graphical representation showing the prepended informationwhen the equalizer prepend bit is set, thereby increasing the prependedinformation by 32 bytes (for a total length of 48 bytes) with respect toFIG. 52;

FIG. 54 is a table showing the statistics and the calculation used foreach slot definition;

FIG. 55 is a block diagram of the MAC/PHY interface;

FIG. 56 is a graphical representation showing the relationship of thebit clock with respect to the burst valid indicator (BlkDV) and thedata;

FIG. 57 is a graphical representation showing the MAP serial interfacefield definitions;

FIG. 58 is a graphical representation showing the format of theprepended data;

FIG. 59 shows the signaling for the data/control MAC/PHY interface atthe subscriber cable modem;

FIG. 60 is a block diagram showing the sign-on sequence for the cablemodem initialization process;

FIG. 61 is a block diagram showing the relationship of the cable modemto the cable modem termination system;

FIG. 62 is a graphical representation showing the contents of theprepended information;

FIG. 63 is a table showing the definitions of the bit fields for thestatus bytes in the prepended information;

FIG. 64 is a block diagram showing the burst demodulator statusinformation processing flow;

FIG. 65 is a block diagram showing the burst detector SPI bus interface;

FIG. 66 is a timing diagram showing one mode of the generic byte baseserial input with control information prepended;

FIG. 67 is a timing chart showing another mode the generic byte baseserial input with control information prepended;

FIG. 68 is a schematic diagram showing the fragmentation of a datapacket of a cable modem into first and second portions thereof, whereinthe first portion of the data packet is placed in a first time slotallocated by the cable modem termination system and the second portionof the data packet is placed in a second time slot allocated by thecable modem termination system;

FIG. 69 is a schematic diagram of a complete packet according to thepresent invention, which is used to transmit data from a cable modem toa cable modem termination system;

FIG. 70 is a schematic diagram of a plurality of complete packetsaccording to the present invention, used to transmit data on aconcatenated basis from a cable modem to the cable modem terminationsystem;

FIG. 71 is a schematic diagram of a plurality of packet fragmentstransmitted from a cable modem to the cable modem termination system,wherein the packet fragments form, in composite, a complete packet;

FIG. 72 shows the format of one of the packets of FIG. 71 in furtherdetail;

FIG. 73 and FIG. 74, taken together, define a table providing furtherdetail of the fragmentation format of a frame which incorporates apacket;

FIGS. 75 and 76, taken together, define a flowchart showing how a cablemodem and a cable modem termination system cooperate to facilitate thefragmentation of packets by the cable modem for transmission to thecable modem termination system;

FIG. 77 is a flowchart illustrating the fragmentation process;

FIG. 78 is a modification of FIG. 1 adapting the invention to wirelesstransmission;

FIG. 79 is a modification of FIG. 2 adapting the invention to wirelesstransmission;

FIG. 80 is a schematic diagram of a single integrated circuit chipadapted to practice the invention;

FIG. 81 is a schematic block diagram, of a bidirectional cabletransmission system;

FIG. 82 is a schematic block diagram of a portion of the RF receiver atthe headend of the cable system shown in FIG. 81;

FIG. 83 is a schematic block diagram of the adaptive notch filter shownin FIG. 82;

FIG. 84 is a schematic block diagram of the generalized decisionfeedback equalizer (DFE) shown in FIG. 82;

FIG. 85 is a schematic block diagram of a portion of one of the cablemodems shown in FIG. 81;

FIG. 86 is a diagram of the TDMA slots for transmitting information inan upstream channel of the cable system shown in FIG. 81;

FIG. 87 is a block diagram of a method for reducing noise in the cablesystem shown in FIG. 81;

FIGS. 88A-88C are frequency response diagrams illustrating common noise(such as ingress) cancellation according to the method shown in FIG. 87;

FIGS. 89A and 89B are diagrams of a 16-QAM constellation before andafter noise cancellation according to the method shown FIG. 87; and

FIGS. 90A and 90B are frequency response diagrams illustrating bothingress and individual noise compensation according to the method shownin FIG. 87.

DETAILED DESCRIPTION OF THE INVENTION Introduction

In a cable modem system, a headend or cable modem termination system(CMTS) is located at a cable company facility and functions as a modemwhich services a large number of subscribers. Each subscriber has acable modem (CM). Thus, the cable modem termination system must becapable of facilitating bidirectional communication with any desired oneof the plurality of cable modems.

As used herein, the cable modem termination system (CMTS) is defined toinclude that portion of a headend which facilitates communication with aplurality of cable modems. A typical cable modem termination systemincludes a burst receiver, a continuous transmitter and a medium accesscontrol (MAC).

The cable modem termination system communicates with the plurality ofcable modems via a hybrid fiber coaxial (HFC) network, wherein opticalfiber provides communication to a plurality of fiber nodes and eachfiber node typically serves approximately 500 to 2,000 subscribers,which communicate with the node via coaxial cable. A plurality ofsubscribers communicate with the fiber node via a common or sharedcoaxial cable. It is this sharing of the common coaxial cable whichnecessitates that the number of cable modems attached thereto be limitedso as to mitigate the likelihood of undesirable bit rate reductionswhich inherently occur when an excessive number of cable modemscommunicate simultaneously over a single coaxial cable.

The hybrid fiber coaxial network of a cable modem system utilizes apoint-to-multipoint topology to facilitate communication between thecable modem termination system and the plurality of cable modems.Frequency domain multiple access (FDMA)/time division multiplexing (TDM)is used to facilitate communication from the cable modem terminationsystem to each of the cable modems, i.e., in the downstream direction.Frequency domain multiple access (FDMA)/time domain multiple access(TDMA) is used to facilitate communication from each cable modem to thecable modem termination system, i.e., in the upstream direction.

The cable modem termination system includes a downstream modulator forfacilitating the transmission of data communications therefrom to thecable modems and an upstream demodulator for facilitating the receptionof data communications from the cable modems.

The downstream modulator of the cable modem termination system utilizeseither 64 QAM or 256 QAM in a frequency band of 54 MHz to 860 MHz toprovide a data rate of up to 56 Mbps.

Since the upstream channel has a much lower data rate requirement, theupstream demodulator uses either QPSK or 16 QAM in a frequency range of5 MHz to 42 MHz to provide a data rate of up to 10 Mbps.

The asymmetric data throughput defined by the upstream channel requiringa much lower data rate than the downstream channel results from theinherently larger amount of data which is communicated via thedownstream channel during pay-per-view, Internet access and the like,wherein a video signal is communicated via the downstream channel, whileonly control signals such as those associated with viewing of the videosignal are communicated via the upstream channel. Thus, the downstreamchannel requirement may exceed 1.5 Mbps, while the upstream channelrequirement may be as low as 16 Kbps.

Similarly, each cable modem includes an upstream modulator forfacilitating the transmission of data to the cable modem terminationsystem and a downstream demodulator for receiving data from the cablemodem termination system. The upstream modulator of each cable modemuses either QPSK or 16 QAM within the 5 MHz to 42 MHz bandwidth of theupstream demodulator and the downstream demodulator of each cable modemutilizes either 64 QAM or 256 QAM in the 54 MHz to 860 MHz bandwidth ofthe downstream modulator (in North America).

Contemporary cable modem systems operate on a plurality of upstreamchannels and utilize time division multiple access (TDMA) in order tofacilitate communication between a plurality of cable modems and asingle cable modem termination system on each upstream channel.Typically, between 250 and 500 cable modems communicate with a singlecable modem termination system on a given upstream channel.

In order to accomplish TDMA for upstream communication, it is necessaryto assign time slots within which cable modems having a message to sendto the cable modem termination system are allowed to transmit. Theassignment of such time slots is accomplished by providing a requestcontention area in the upstream data path within which the cable modemsare permitted to contend in order to place a message which requestsadditional time in the upstream data path for the transmission of theirmessage. The cable modem termination system responds to these requestsby assigning time slots to the cable modems making such a request, sothat as many of the cable modems as possible may transmit their messagesto the cable modem termination system utilizing TDMA and so that thetransmissions are performed without undesirable collisions.

Because of the use of TDMA, the cable modem termination system must usea burst receiver, rather than a continuous receiver, to receive datapackets from cable modems via upstream communications. As those skilledin the art will appreciate, a continuous receiver can only be utilizedwhere generally continuous communications (as opposed to burstcommunications as in the present invention) are performed, so as tosubstantially maintain timing synchronization between the transmitterand the receiver, as is necessary for proper reception of thecommunicated information. During continuous communications, timingrecovery is a more straightforward process since signal acquisitiongenerally only occurs at the initiation of such communications. Thus,acquisition is generally only performed in continuous receivers once percontinuous transmission and each continuous transmission may be verylong.

However, the burst communications inherent to TDMA systems requireperiodic and frequent reacquisition of the signal. That is, during TDMAcommunications, the signal must be reacquired for each separate bursttransmission being received.

Since continuous receivers generally only acquire the signal once, theneed to minimize acquisition time is much less critical in continuousreceivers than in burst receivers, wherein acquisition must be performedfor each separate burst, and therefore occurs quite frequently. Thus,there is a strong motivation to minimize acquisition time in burstreceivers, so as to enhance overall data transmission efficiency andthroughput. As such, it is beneficial to provide techniques whichenhance the speed at which data packets transmitted according to TDMAmethodologies may be acquired by a burst receiver, such as that of acable modem termination system.

Burst Receiver for Cable Modem System and Synchronization

Referring now to FIG. 1, a hybrid fiber coaxial (HFC) network 1010facilitates the transmission of data between a headend 1012, whichincludes at least one cable modem termination system, and a plurality ofhomes 1014, each of which contains a cable modem. Such hybrid fibercoaxial networks are commonly utilized by cable providers to provideInternet access, cable television, pay-per-view and the like tosubscribers.

Approximately 500 homes 1014 are in electrical communication with eachnode 1016, 1034 of the hybrid fiber coaxial network 1010, typically viacoaxial cables 1029, 1030, 1031. Amplifiers 1015 facilitate theelectrical connection of the more distant homes 1014 to the nodes 1016,1034 by boosting the electrical signals so as to desirably enhance thesignal-to-noise ratio of such communications and by then transmittingthe electrical signals over coaxial cables 1030, 1031. Coaxial cable1029 electrically interconnects the homes 1014 with the coaxial cables1030, 1031, which extend between amplifiers 1015 and nodes 1016, 1034.

Each node 1016, 1034 is electrically connected to a hub 1022, 1024,typically via an optical fiber 1028, 1032. The hubs 1022, 1024 are incommunication with the headend 1012, via optical fibers 1020, 1026. Eachhub is typically capable of facilitating communication withapproximately 20,000 homes 1014.

The optical fibers 1020, 1026 extending intermediate the headend 1012and each hub 1022, 1024 defines a fiber ring which is typically capableof facilitating communication between approximately 100,000 homes 1014and the headend 1012.

The headend 1012 may include video servers, satellite receivers, videomodulators, telephone switches and/or Internet routers 1018, as well asthe cable modem termination system. The headend 1012 communicates viatransmission line 1013, which may be a T1 or T2 line, with the Internet,other headends and/or any other desired device(s) or network.

Referring now to FIG. 2, a simplified block diagram shows theinterconnection of the headend 1012 and an exemplary home 1014, whereina cable modem 12 communicates with a cable modem termination system,embodied as a line card 1042, via hybrid fiber coaxial network 1010.

More particularly, a personal computer 1048, disposed within the home1014, is connected via cable 1011 to the cable modem 12 whichcommunicates via coaxial cable 1017 with the hybrid fiber coaxialnetwork 1010, which in turn communicates via optical fiber 1020 with thecable modem termination system (CMTS) including line card 1042 of theheadend 1012. Internet router 1040 facilitates communication between theheadend 1012 and the Internet or any other desired device or network.

Referring now to FIG. 3, the present invention includes a cable modemtermination system (defined by line card 1042 of FIG. 2) whichcommunicates with a plurality of cable modems 12. Cable modemtermination system (CMTS) 10 has an enhanced data packet acquisitionburst receiver 580. Burst receiver 580 includes an analog front-end suchas an analog-to-digital converter 582 which receives analog data packetsfrom an upstream channel and which converts the analog data packets intodigital data packets, a fractional symbol timing loop 584 whichdetermines a fractional symbol timing correction and applies thefractional symbol timing correction to the data packets, a carrier phasecorrection loop 586 which determines a carrier phase correction andapplies the carrier phase correction to the data packets, a phasederotator 588 which corrects phase errors in the symbols of the datapackets, and a conventional coherent amplitude estimator 590 whichprovides an amplitude correction by a conventional estimation processand applies the amplitude correction to the data packets via multiplier592 prior to the data packets being provided to slicer 594. This processis described in detail below.

The timestamp generation at the CMTS and the upstream timing recoverylogic at the CM, and the flow of timestamp message are shown in FIG. 6A.Although only one cable modem 12 is shown in FIG. 6A for clarity, thecable modem termination system 10 actually communicates bidirectionallywith a plurality of such cable modems 12. Such communication asdiscussed herein may actually occur between the cable modem system andthe plurality of cable modems by communicating simultaneously with thecable modems on a plurality of separate frequency channels.

This aspect of the invention primarily addresses communication of aplurality of different cable modems on a single frequency channel in aserial or time division multiplexing fashion, wherein the plurality ofcable modems communicate with the cable modem termination systemsequentially. However, it will be appreciated that while this pluralityof cable modems is communicating on one channel with the cable modemtermination system (using time division multiple access or TDMA), manyother cable modems may be simultaneously communicating with the samecable modem termination system on a plurality of different channels(using frequency division multiplexing/time division multiple access orFDM/TDMA).

In a typical cable modem system, a single cable modem termination systemincluding line card 1042 (FIG. 2) will typically communicate withbetween 250 and 500 cable modems 12. Thus, the cable modem system of thepresent invention includes a plurality of cable modems 12. Although thefollowing description generally discusses the operation of a singlecable modem termination system including line card 1042 and a singlecable modem 12, those skilled in the art will appreciate that aplurality of cable modem termination systems including line cards 1042and cable modems 12 may similarly be utilized.

The cable modem termination system 10 communicates with each of thecable modems 12 via a cable plant 8, which typically includes a hybridfiber coaxial (HFC) network in which optical fiber facilitatescommunication from the cable modem termination system 10 to a pluralityof hubs, each of which distribute signals from the optical fiber to aplurality of coaxial cables. Each hub may be located at a distance of upto approximately 100 miles from either the cable modem terminationsystem 10 or from the next hub along the optical fiber.

Optionally, a plurality of cable modem termination systems may besynchronized with respect to one another so as to facilitatecommunication between any desired cable modem termination system and anydesired cable modem(s).

According to one aspect of the present invention, the cable modemtermination system 10 includes a crystal oscillator timing reference 16which provides an output to a linear counting sequence generator 21. Itis this timing reference 16 to which each of the cable modems 12 must besynchronized. The linear counting sequence generator 21 is incrementedby the output of the crystal oscillator timing reference 16 andmaintains a count representative of the number of cycles provided by thecrystal oscillator timing reference 16 since the linear countingsequence generator 21 was last reset. According to the presentinvention, the linear counting sequence generator 21 includes afree-running counter having a sufficient count capacity to count forseveral minutes before resetting.

A timebase message (timebase message and timestamp message are usedinterchangeably herein) generator 20 receives the count of the linearcounting sequence generator 21 to provide an absolute time referencewhich is inserted into the downstream information flow 23 provided bydownstream data queue 24, as discussed in detail below. The timebasemessage generator 20 performs a modulo function, i.e., a sawtoothpattern as a function of time, and the counter clock is generated by theoscillator with very tight accuracy.

Slot timing offset generator 26 receives a timing offset (rangingsignal) message 27 from each individual cable modem 12 with which thecable modem termination system is in communication. The slot timingoffset generator 26 provides a slot timing offset 28 which isrepresentative of a slot timing offset between the cable modemtermination system 10 and the cable modem 12 and inserts the slot timingoffset 28 into the downstream information flow 23. The slot timingoffset 28 is calculated by determining the position of the slot timingoffset from the expected time of message 27 within a dedicated timingslot of the upstream communications, as discussed in detail below. Thetiming offset generator 26 encodes the timing offset (ranging error)detected by the upstream receiver into a slot timing offset message.

Slot timing offset messages are sent only after the frequency of thelocal reference clock has been acquired by the cable modem.

Downstream modulator 30 primarily modulates the downstream informationflow 23. Absolute time references are inserted at quasi-periodicintervals as determined by a timestamp send timer in the form of abinary up counter 31 (FIG. 6C). A slot timing offset 28 is insertedshortly after the arrival of a slot timing offset message 27.

The time line 32 of the cable modem termination system 10 shows that theslot timing offset 28 is the difference between the expected receivetime and the actual receive time of the slot timing offset message 27.

According to one embodiment of the present invention, each cable modem12 includes a downstream receiver 15 for facilitating demodulation ofthe data and timestamp message, and timing recovery of downstreamcommunications from the cable modem termination system 10. The output ofthe downstream receiver 15 is provided to timebase message detector 36and slot timing offset detector 38. The downstream information (any datacommunication, such as a file transfer or MPEG video signal) received bythe downstream receiver 15 is also available for further processing, asdesired.

The timebase message detector 36 detects the timebase message generatedby timebase message generator 20 of the cable modem termination system10. Similarly, the slot timing offset detector 38 detects the slottiming offset 28 generated by the slot timing offset generator 26 of thecable modem termination system 10. The timebase message detector 36provides an absolute time reference which is representative of thefrequency of the crystal oscillator timing reference 16 of the cablemodem termination system 10. The absolute time reference is provided toa digital tracking loop 42 which provides a substantially stable clockoutput for the cable modem 12 which corresponds closely in frequency tothe frequency of the crystal oscillator timing reference 16 of the cablemodem termination system 10. Thus, the digital tracking loop 42 uses theabsolute time reference, which is representative of the frequency of thecrystal oscillator timing reference 16, to form an oscillator drivesignal which drives a numerically controlled oscillator 44 in a mannerwhich closely matches the frequency of the crystal oscillator timingreference 16 of the cable modem termination system 10, as discussed indetail below.

A difference between the absolute time reference and the output of alocal time reference 46, which is derived from the numericallycontrolled oscillator 44, is formed by a differencer 48. This differencedefines a frequency error value which represents the difference betweenthe clock of the cable modem 12 (which is provided by local timereference 46) and the clock of the cable modem termination system 10(which is provided by crystal oscillator timing reference 16).

This frequency error value is filtered by loop averaging filter 50 whichprevents undesirable deviations in the frequency error value fromaffecting the numerically controlled oscillator 44 in a manner whichwould decrease the stability thereof or cause the numerically controlledoscillator 44 to operate at other than the desired frequency. The loopfilter 50 is configured so as to facilitate the rapid acquisition of thefrequency error value, despite the frequency error value being large,and then to reject comparatively large frequency error values as thedigital tracking loop 42 converges, i.e., as the output of the localtiming reference 46 becomes nearly equal to the absolute time reference,thereby causing the frequency error value to approach zero.

According to one embodiment of the present invention, an initial slottiming offset 52 is added by summer 61 to the output of the local timereference 46 to provide a partially slot timing offset corrected output56. The partially slot timing offset corrected output 56 of summer 61 isthen added to slot timing offset 58 provided by slot timing offsetdetector 38 to provide slot timing offset and frequency corrected timereference 86. The timing offset correction is a simple addition whichadds two message values. Such simplified operation is facilitated onlywhen the resolution of the timing offset message is equal to or finerthan that of the timestamp message.

The initial slot timing offset 52 is merely an approximation of theexpected slot timing offset likely to occur due to the propagation andprocessing delays, whose approximate values have been predetermined.After frequency conversion using the phase locked loop and timebasemessage error, the slot timing offset 58 provides a final correctionwhich is calculated by the cable modem termination system 10 in responseto the cable modem termination system 10 receiving communications fromthe cable modem 12 which are not properly centered within their desiredtiming slots, as discussed in detail below.

Scaler 87 scales the frequency corrected time reference 86 so as todrive upstream transmitter 69 at the desired slot timing.

Time reference 88 is compared to the designated transmit time 89 whichwas allocated via downstream communication from the cable modemtermination system 10 to the cable modem 12. When the time reference 88is equal 67 to the designated transmit time, then an initiate burstcommand 65 is issued and the upstream data queue 71 is modulated to formupstream transmission 75.

The timing offset (error) message is generated by the cable modemtermination system. The timing offset (error) is simply the differencebetween the expected time and the actual arrival time of the messageduring the ranging slot at the cable modem termination system receiver.

Referring now to FIG. 6B, the cable modem termination system 10 and thecable modem 12 are described in further detail. The multiplexer 29 ofthe cable modem termination system 10 combines downstream informationflow 23 with slot timing offset message 28 from slot timing offsetgenerator 26 and with an absolute time reference from timebase messagegenerator 20 to provide downstream communications to the downstreamtransmitter, which includes downstream modulator 30 (FIG. 6A).

The slot timing offset generator 26 receives a slot timing offset signal77 from the upstream receiver 13. The location of the slot timing offsetsignal within a time slot of an upstream communication defines the need,if any, to perform a slot timing offset correction. Generally, a slottiming offset value will be transmitted, even if the actual slot timingoffset is 0. When the slot timing offset signal is desirably locatedwithin the time slot, and does not extend into guard bands which arelocated at either end of the time slot, then no slot timing offsetcorrection is necessary.

However, when the slot timing offset signal extends into one of theguard bands of the time slot of the upstream communication, then a slottiming offset message 28 is generated by the slot timing offsetgenerator 26, which is transmitted downstream to the cable modem 12where the slot timing offset message 28 effects a desired correction tothe time at which upstream communications occur, so as to cause the slottiming offset signal and other transmitted data to be positionedproperly within their upstream time slots.

The headend tick clock 25 includes the crystal reference 16 of FIG. 6Aand provides a clock signal to linear counting sequence generator 21.Slot/frame time generator 19 uses a clock signal provided by countsequence generator 21 to provide both a minislot clock 19 a and areceive now signal 19 b. The absolute time reference from generator 20is the clock by which the message slots are synchronized to effect timedivision multiple access (TDMA) communications from each cable modem 12to the cable modem termination system 10. At the CM, a Transmit nowsignal is generated at the beginning of each minislot of a transmission(FIG. 61). At the CMTS, a Receive now signal is similarly generated atthe beginning of a received packet (FIGS. 49-51).

A minislot is a basic medium access control (MAC) timing unit which isutilized for allocation and granting of time division multiple access(TDMA) slots. Each minislot may, for example, be derived from the mediumaccess control clock, such that the minislot begins and ends upon arising edge of the medium access control clock. Generally, a pluralityof upstream symbols define a minislot and a plurality of minislotsdefine a time division multiple access slot.

The cable modem 12 receives downstream data from the downstream channel8B. A timebase message detector 36 detects the presence of a timebasemessage in the downstream data.

Slot timing offset correction 47 is applied to data transmitted onupstream channel 8A prior to transmission thereof from the subscribercable modem 12. The slot timing offset correction is merely thedifference between the actual slot timing offset and the desired slottiming offset. Thus, the slot timing offset correction is generatedmerely by subtracting the actual slot timing offset from the desiredoffset. Slot/frame timing generator 63 controls transmission of theupstream data queue 71 (FIG. 6A) at the designated transmit time 89(FIG. 6A).

Summer 48 subtracts the local time reference 46 from the timebasemessage and provides an output to a loop filter 50 which drivesnumerically controlled oscillator 44, as discussed in detail below.

Upstream transmitter 11 facilitates the transmission of upstreamchannels 8A from the subscriber cable modem 12 and upstream receiver 13facilitates the reception of the upstream channels 8A by the cable modemtermination system 10.

Downstream transmitter 17 facilitates the transmission of downstreamchannels 8B from the cable modem termination system 10 to the cablemodem 12 where downstream receiver 15 facilitates reception thereof.

Referring now to FIG. 6C, the cable modem termination system 10 is shownin further detail.

As discussed above, the crystal oscillator timing reference 16 providesan output to linear counting sequence generator 21 which increments toprovide a count representative of the frequency of the crystaloscillator timing reference 16. The counter 21 also provides asubstantially jitterless headend reference which provides a clock signalfor downstream data transmissions from the cable modem terminationsystem 10. The jitterless headend reference is synchronized to thedownstream symbol rate via synchronizer 37 which includes counters 412,413, 414, inverter 93 and AND gate 94 which cooperate according towell-known principles to provide a timestamp latch enable to AND gate 39to enable latch 41.

The linear counting sequence generator 21 provides its count to latch41. Latch 41 provides the count from the linear counting sequencegenerator 21 to multiplexer 45 when an enable is provided to latch 41.The enable is provided to latch 41 when the synchronizer 37 provides ahigh output and the downstream processor 718 provides a low output toAND gate 39. The count from the linear counting sequence generator 21 iscombined with a timebase message header 43 by multiplexer 45 and thecombined count and timebase message header is provided to the downstreamprocessor 718. The downstream processor 718 provides a control signal tothe multiplexer 45 to cause the multiplexer 45 to provide the count fromthe linear counting sequence generator 21 and the timebase messageheader 43 to the downstream processor 718 only when the downstreamprocessor 718 is ready to insert the count and the timebase messageheader 43 into a downstream data communication.

Binary up counter 31 functions as a timestamp send timer so as to causethe count or absolute time reference (FIG. 6A) from the linear countingsequence generator 21 to be inserted into a downstream communication ina generally periodic fashion. The binary up counter 31 receives a countfrom the linear counting sequence generator 21. When the count of thebinary up counter 31 equals a value stored in the threshold register 33,equality comparitor 35 provides a request timestamp send to thedownstream processor 718. It is important to note that the timestampincludes the absolute time reference (FIG. 6A).

However, the downstream processor 718 does not immediately insert everycombined count and timebase message header from multiplexer 45 into adownstream communication when the request timestamp send 59 is providedby the equality comparitor 35 to the downstream processor 718. Rather,the downstream processor 718 waits until any downstream messagepresently being transmitted is finished so as to prevent undesirablefragmentation thereof.

The downstream processor 718 provides downstream data, includingdownstream communications from the downstream data queue 24, a countfrom the linear counting sequence generator 21, and a timebase messageheader 43 from multiplexer 45 to the downstream modulator 51, whichmodulates the data, count, and timebase message header to form adownstream data communication 53 which includes a plurality ofindividual messages 55. Some of these individual messages 55 includescommunicated data such as file transfers and MPEG video and some ofthese messages 55 include timestamps and/or slot timing offsets tofacilitate synchronization of a selected cable modem 12 with the cablemodem termination system 10.

In this manner, a count which is representative of the frequency of thecrystal oscillator timing reference 16 is transmitted from the cablemodem termination system 10 to each cable modem 12.

The output of the linear counting sequence generator 21 is divided downto provide a frequency reduced slow tick clock output signal.

Referring now to FIG. 6D, an exemplary timing recovery circuit of acable modem is shown in further detail. Downstream demodulator 95, whichforms a portion of downstream receiver 15 of FIG. 6B, provides clock anddata signals which are derived from downstream channels 8B (FIG. 6A).The data signals include downstream bytes which in turn include thecount or timestamp 97 and timebase message header 81 transmitted by thecable modem termination system 10. Slot timing offset messages areincluded in the downstream flow of downstream data.

Timestamp detector 80 detects the presence of a timestamp header 81among the downstream bytes and provides a timestamp arrived signal 82which functions as a downstream byte clock sync. The timestamp arrivedsignal 82 is provided to synchronizer 83 which includes register 101,register 102, AND gate 103, inverter 104 and latch 105, which stretchesthe input and generates a tick clock synch pulse 107. Synchronizer 83synchronizes the timestamp arrived signal 82 to the clock of the cablemodem 12, to provide a data path enable tick clock sync pulse 107 forenabling the digital tracking loop 42.

When the digital tracking loop 42 is enabled by the pulse 107 from thesynchronizer 83 in response to detecting a timestamp header by timestampdetector 80, then the timestamp, which is a count provided by the linearcounting sequence generator 21 of FIG. 6C, is provided to the digitaltracking loop 42 and the digital tracking loop 42 is enabled so as toprocess the timestamp.

A differencing circuit or saturating frequency detector 109 compares thetimestamp to a count provided to the saturating frequency detector 109by timebase counter 111 which is representative of the frequency ofnumerically controlled oscillator 44. The saturating frequency detector109 provides a difference signal or frequency error value 112 which isproportional to the difference between the frequency of the numericallycontrolled oscillator 44 of the cable modem and the crystal oscillatorreference 16 of the cable modem termination system.

If the difference between the timestamp and the value of the timebasecounter 111 is too large, then the difference is saturated to a maximumor minimum level depending on the sense of the excessive difference.

Detector 109 is coupled by a zero or pass connection 113 to latch 115.Responsive to a loop enable signal, the difference provided by thedetector 109 is provided to latch 115 when a global enable is providedthereto. The loop enable is set active when functioning of the digitaltracking loop is desired.

Latch 115 provides the frequency error value 112 to a loop filter whichincludes multipliers 117 and 119, scalers 121 and 123, summers 124, 125and latch 127.

The multipliers 117 and 119 include shift registers which effectmultiplication by shifting a desired number of bits in either direction.Scalers 121 and 123 operate in a similar manner. The loop filterfunctions according to well-known principles to filter out undesirablefrequency error values, such that they do not adversely affect thestability or operation of numerically controlled oscillator 44. Thus,the loop filter tends to smooth out undesirable deviations in thefrequency error value signal, so as to provide a more stable drivesignal for the numerically controlled oscillator 44.

According to one embodiment of the present invention, the multipliers117 and 119 can be loaded with different coefficients such that thebandwidth of the loop filter may be changed from a larger bandwidthduring initial acquisition to a smaller bandwidth during operation. Thelarger bandwidth used initially facilitates fast acquisition by allowingfrequency error values having larger deviations to be accepted. As thedigital tracking loop 42 converges, the frequency error value tends tobecome smaller. At this time, frequency error values having largerdeviations would tend to decrease stability of the digital tracking loop42 and are thus undesirable. Therefore, different coefficients, whichdecrease the bandwidth of the loop filter, are utilized so as tomaintain stability of the digital tracking loop 42.

A table showing an example of coarse and fine coefficients K0 and K1which are suitable for various different update rates and bandwidths areshown in FIG. 6E.

The output of the loop filter is provided to latch 129. The output oflatch 129 is added to a nominal frequency by summer 133 so as to definea drive signal for numerically controlled oscillator 44.

Those skilled in the art will appreciate that the addition of afrequency offset, if properly programmed to a normal frequency, willdecrease the loop's acquisition time. This is due to the fact that thefinal value of the accumulated value of latch 127 will be closer to itsinitial value.

The nominal frequency is generally selected such that it is close invalue to the desired output of the numerically controlled oscillator 44.Thus, when the numerically controlled oscillator 44 is operating at thedesired frequency, the filtered frequency error value provided by latch129 is nominally zero.

Referring now to FIG. 12, a flowchart showing the two levels of control,i.e., coarse lock and fine lock, of the digital tracking loop 42 isprovided. As mentioned above, the coarse lock utilizes coefficients forthe multipliers 117 and 119 which provide a large bandwidth of the loopfilter which is suitable for the acquisition of the frequency errorvalue so as to initiate tracking, while the fine coefficients provideenhanced stability of the numerically controlled oscillator 44, so as toprevent undesirable fluctuations in the output thereof. According to oneexemplary embodiment of the present invention, a hardware control level,i.e., utilizing coarse coefficients for the multipliers 117 and 119,achieves a coarse frequency lock and then a software level changes theloop coefficients to achieve a final, low jitter frequency lock. At thehardware level, a state of frequency lock implies that the differencebetween arriving timebase message values and the clock, i.e., output ofthe numerically controlled oscillator 44, of the cable modem 12 is belowa predetermined or programmable error threshold. Software lock impliesthat a final low jitter lock state has been achieved.

The process for achieving coarse frequency lock or (sync=1) is nowdescribed. After starting 200, the cable modem 12 first waits 201 forthe loop or data path enable 107 (FIG. 6D) before becoming active. Afterthe first timebase message arrives, then the first timebase message 202is loaded 203 into the timebase counter 111 (FIG. 6D) of the digitaltracking loop 42. This allows the digital tracking loop 42 to beinitiated with a value which produces a zero frequency error value, soas to facilitate faster acquisition and prevent undesirable swings inthe output of the numerically controlled oscillator 44. Thus, when thedata path enable 107 is detected by the digital tracking loop 42, thenthe next arriving timestamp is loaded into the timebase counter of thecable modem 12 and the digital tracking loop 42 then waits 204 for thenext timebase message to arrive.

Loading 203 of the initial timebase message into the timebase counter111 enhances acquisition time because it forces the counter 111 of thecable modem 12 to have a value close to that of the linear countingsequence generator 21 of the cable modem termination system 10. When thenext timebase message arrives 204, the number of messages that havearrived thus far are compared 205 to a programmable threshold. If thenumber of messages (acquisition count) is less than the programmablethreshold (acquisition threshold), then the acquisition count isincremented 206. If the number of messages received so far (acquisitioncount) is greater than the programmable threshold (acquisitionthreshold), then the current timebase error is checked 207 against anerror threshold to determine whether or not sync can be declared(sync=1). If the timebase error is below the threshold, then hardwarecoarse lock has been achieved and sync becomes active. The cable modem12 then waits for the next timebase message to arrive. If the newtimebase error exceeds the error threshold, then the cable modem 12returns to the initial or start state 200, resets the acquisition countand the loop integrator value, i.e., the value stored in latch 127, andthe acquisition process begins again.

Referring now to FIG. 13, the software level of control occurs within alocal processor and affects the digital tracking loop 42 via registerwrites to the loop filter's linear and integrator coefficients. The loopfilter's linear coefficient is that coefficient placed in multiplier 117and the loop filter's integrator coefficient is that coefficient placedin multiplier 119. Loading different sets of coefficients into a loopfilter changes the loop filter's bandwidth, as discussed above.

Thus, coarse coefficients give the digital tracking loop 42 a relativelylarge bandwidth, which enables quick acquisition of frequency errorvalues, while narrower loop bandwidths reject frequency error valuesrepresentative of noisier variations in the error metric, therebysmoothing the digital tracking loop's 42 response. It is important tonote that smooth response of the digital tracking loop 42 is importantin achieving low jitter between the cable modem 12 and the cable modemtermination system 10.

According to the present invention, before enabling the hardwareacquisition control, the first stage of software acquisition controlincludes estimation 300 of the timestamp interarrival time, which isparticularly estimated by averaging the timestamp interarrival time overa plurality, e.g., 10 to 50 arrivals. This estimation is importantbecause the coarse and fine coefficients are obtained from the tableshown in FIG. 6E, where they are dependent upon the update rate, i.e.,timestamp interarrival time.

After interarrival time is estimated as represented by a block 300, thenthe software controller enters an initialization state as represented bya block 301 wherein a trial counter (which counts the number ofacquisition attempts thus far) is reset, the tracking loop 42 isdisabled and the latch 127 of the loop integrator is reset.

Next, the trial counter is incremented and checked as represented by ablock 303 to see if the number of acquisition attempts is less then apredetermined threshold. If the threshold is exceeded, then thecontroller takes a NO path back to block 300 and performs interarrivalestimation again. Otherwise, the controller takes a YES path and thecoarse coefficients are loaded as represented by a block 305 into themultipliers 117 and 119 of the digital tracking loop 42 and the loop isenabled. The software controller then waits as represented by a block307 for the same number of timestamps to arrive as does the hardwarecontroller, after which the sync bit that comes from the hardwarecontroller is checked as represented by a block 309 to determine whetheror not coarse lock has been achieved.

If sync is active (is equal to 1), then fine loop coefficients areloaded as represented by a block 310 and a programmable amount of timeis allowed to lapse as represented by a block 311 before a sync bit ischecked once again. As represented by a block 313, the track errorthreshold value is loaded. The track error threshold is used todetermine whether or not the tracking loop 42 is receiving timestampssuitable for updating the frequency of the numerically controlledoscillator 44. As represented by a block 315 fine lock is checked. Iffine lock is achieved the trial count is incremented via a YES path backto block 315 and further attempts to reacquire force lock can be made.Failure of fine lock causes a loop back to block 301 via a NO path andthe acquisition process is restarted and also resets the trial counter.It is assumed that if fine lock has beep achieved, then the interarrivalestimation should be accurate.

The slot timing offset is determined by having the cable modemtermination system 10 monitor a dedicated slot timing offset slot inupstream communications so as to determine the position of a slot timingoffset message therein. The position of the slot timing offset messagewithin the dedicated slot timing offset slot in the upstreamcommunication determines the slot timing offset between the clock of thecable modem termination system 10 and the clock of the cable modem 12.Thus, the cable modem termination system 10 may use this error to causethe cable modem 12 to transmit at an earlier point in time so as tocompensate for propagation and processing delays. As illustrated in FIG.11, this slot timing offset correction is equal to 2Tpg plus Tprocess.

Initially, the slot timing offset slot includes a comparatively largetime slot, i.e., having comparatively large guard times, so as toaccommodate comparatively large slot timing offset error. In a normaldata packet, the width of the timing offset slot may be reduced whenslot timing offset errors become lower (thus requiring smaller guardbands), so as to facilitate more efficient upstream communications.

Generally, communications will be initialized utilizing a comparativelylarge guard time. After acquisition, when slot timing accuracy has beenenhanced, then the guard time may be reduced substantially, so as toprovide a corresponding increase in channel utilization efficiency.

According to a further aspect of the present invention, data packets areacquired rapidly, e.g., in an order of sixteen symbol or so, so as tofacilitate enhanced efficiency of bandwidth usage. As those skilled inthe art will appreciate, it is desirable to acquire data packets as fastas possible, so as to minimize the length of a header, preamble or othernon-information bearing portion of the data packet which is usedexclusively for such acquisition.

As used herein, acquisition is defined to include the modifications oradjustments made to a receiver so that the receiver can properlyinterpret the information content of data packets transmitted thereto.Any time spent acquiring a data packet detracts from the time availableto transmit information within the data packet (because of the finitebandwidth of the channel), and is therefore considered undesirable.

According to the present invention, acquisition includes the performanceof fine adjustments to the parameters which are defined or adjustedduring the ranging processes. During the ranging processes, slot timing,carrier frequency, and gross amplitude (power) of the data packet aredetermined. During acquisition, these parameters are fine-tuned so as toaccommodate fractional symbol timing, carrier phase correction and fineamplitude of the data packet.

Moreover, according to the present invention, a ranging process is usedto control power, slot timing and carrier frequency in the upstream TDMAchannel. Power must be controlled so as to provide normalized receivedpower at the cable modem termination system, in order to mitigateinter-channel interference. The carrier frequency must be controlled soas to ensure proper channelization in the frequency domain. Slot timingmust be controlled so as to mitigate the undesirable collision of datapackets in the time domain and to account for differential propagationdelays among different cable modems.

Fractional symbol timing is a precise modification to slot timing. Inslot timing, the clocks of the cable modems are synchronized such that adata packet is transmitted within a slot defined by the cable modemtermination system, so as to avoid collisions of data packetstransmitted simultaneously by different cable modems. Duringacquisition, fractional symbol timing allows the receiver to samplesymbols at the correct time. Thus, fractional symbol timing causes thereceive symbols of the data packet to be aligned in time such that theyare properly demodulated. As those skilled in the art will appreciate,it is important to detect the amplitude of QAM symbols at the correcttime, so as to facilitate proper interpretation of the amplitudethereof.

Carrier phase correction is a fine tuning of carrier frequencycorrection, which is performed during the ranging process. Carrier phasecorrection is necessary in order for the phase derotator to properlycompensate for phase errors in the received packet.

Fine amplitude correction is a more precise correction to grossamplitude correction, which is performed during a ranging process.Amplitude corrections must be applied to the incoming data packet, so asto assure that the amplitude is properly defined prior to amplitudedetection by the slicer.

Thus, according to the present invention, acquiring a data packet in acable modem termination system includes determining fractional symboltiming correction, determining carrier phase correction and determiningfine amplitude correction. According to the present invention,fractional symbol timing correction is determined by a feedback loopprocess, carrier phase correction is determined by a loop process andfine amplitude correction is determined by an estimation process. Unlikeconventional methodology where fractional symbol timing correction,carrier phase correction, and fine amplitude are all determined by anestimation or correlation technique, this architecture can takeadvantages of the following merits: a) the same feedback loops can beused both for acquisition and tracking of symbol timing and carrierphase, and b) the carrier phase acquisition and small frequency offsetcorrection (important during the ranging process) can be performed byusing the second-order loop architecture.

More particularly, the present invention includes determining fractionalsymbol timing correction via a fractional symbol timing phase lockedloop which controls a phase of a signal representative of the datapacket being acquired as the data packet is processed in a resamplerwhich provides an input to a phase derotator and includes determining acarrier phase correction which is performed by a carrier phasecorrection phase locked loop which controls a phase of a signalrepresentative of the data packet being acquired in the phase derotator.In this manner, the fractional symbol timing is controlled as the signalrepresentative of the data packet being acquired is processed by theresampler and the carrier phase is controlled as the derotator performsphase correction.

The first resampler 1154 (shown in FIG. 22) provides a sample ratesuitable for processing by a matched filter and/or the phase derotator.It allows the analog-to-digital converter sample rate and the symbolrate to be independent and also programmable by the resampling factor.Optionally, the matched filter processes the signal representative ofthe data packet being acquired before the resampler which provides asample rate suitable for carrier phase recovery and a separate resampler1146 (shown in FIG. 22) is used to perform fast clock phase recovery, asdiscussed in detail below.

The matched filter compensates for the effects produced by a shapingfilter of the cable modem transmitter which provided the data packetbeing acquired, according to well-known principles.

Thus, according to one aspect of the present invention, the signalrepresentative of the data packet being acquired is processed by a firstresampler to provide a sample rate suitable for the matched-filter.Then, a signal representative of the data packet being acquired isprocessed by the matched filter. Then, the signal representative of thedata packet being acquired is processed by a second resampler to providea sample rate suitable for the phase derotator which the phase of thesignal representative of the data packet being acquired is processed bya phase derotator to effect correction of a phase of either the in-phase(I) or quadrature (Q) channel of a QAM signal while the phase of thesignal representative of the data packet being acquired is controlled bya carrier phase correction phase locked loop. Then, the signalrepresentative of the data packet being acquired is multiplied by anestimated amplitude correction factor to provide a signal suitable forprocessing by the slicer. Then, the signal representative of the datapacket being acquired is processed by the slicer to effect demodulationof an amplitude component of the I or Q channel of the QAM signal.

Optionally, the input gain (or phase detector gain) of the fractionalsymbol timing phase locked loop and/or the carrier phase correctionphase locked loop by sensing an amplitude input to a phase detector ofthe loop and modifying the amplitude of the input to the loop filter.

As those skilled in the art will appreciate, loop filters tend to beamplitude sensitive since the coefficients selected therefor may not bevalid if the input to the phase detector has an amplitude which issubstantially different from that for which the coefficients wereselected. The use of coefficients which are not suitable for the inputamplitude to the phase detector may therefore result in undesirablyincreased acquisition time of the signal being acquired.

According to the present invention, use of the fractional symbol timingfeedback loop and the carrier phase loop with minimum loop delay, aswell as the fine amplitude estimation process, facilitate theacquisition of a data packet having a preamble which is sixteen symbolsor less in length.

According to the present invention, the preamble includes a binarypattern 1111 and a unique word 1112 (FIG. 17). Further, according to thepresent invention, both the binary pattern and the unique word aremodulated using quadrature phase shift keying (QPSK). Thus, the entirepreamble is modulated using QPSK.

The fractional symbol timing and the carrier phase are determined usingthe binary pattern of the preamble. The fine amplitude correction isdetermined using the unique word of the preamble.

According to one aspect of the present invention, as shown in FIG. 24,the fractional symbol timing acquisition is accelerated by utilizing twooffset symbol sampling clocks and selecting that offset symbol samplingclock which provides samples having the highest absolute value at thebeginning of each burst. As those skilled in the art will appreciate,when only a single symbol sampling clock is utilized, the phase of thesymbol sampling clock may be such that samples of the alternating binarypattern are taken at times when the alternating binary pattern is nearthe transition point, i.e., has a value which is approximately zero, andthe sample levels are therefore ambiguous or difficult to reliablydetermine.

Thus, when only one symbol sampling clock is utilized, the phase of thatsingle symbol sampling clock must be varied until the alternating binarypattern is properly acquired. As those skilled in the art willappreciate, varying the phase of the single sampling clock until thealternating binary pattern is properly acquired (has sufficientamplitude) is undesirably time consuming and thus results in a greateracquisition time of the alternating binary pattern of the preamble.

The use of two offset symbol sampling clocks, particularly when the twooffset symbol sampling clocks are offset approximately 180 degrees withrespect to one another, inherently causes one of the two symbol samplingclocks to sample when the amplitude of the alternating binary pattern issufficient to reliably determine the information content thereof. Thus,according to this aspect of the present invention, two offset symbolsampling clocks, each having a phase difference of approximately 180degrees with respect to the other, are utilized and that clock whichprovides the best, e.g., highest amplitude (absolute value) is utilizedin the sampling process for the alternating binary pattern of thepreamble. The use of two offset symbol sampling clocks thussubstantially shortens the acquisition time of the alternating binarypattern.

Referring now to FIG. 4, the cable modem termination system 1042(typically defined by the line card of FIG. 2) comprises a burstreceiver 292 for receiving data packets in the upstream data flow, acontinuous transmitter 290 for broadcasting to the cable modems 12 viathe downstream data flow and a medium access control (Headend MAC) 60for providing an interface between the burst receiver 292, thecontinuous transmitter 290 and other headend communications devices suchas video servers, satellite receivers, video modulators, telephoneswitches and Internet routers 1018 (FIG. 1).

Each cable modem 12 (FIG. 2) comprises a burst transmitter 294 fortransmitting data to the cable modem termination system including linecard 1042 via downstream data flow, a continuous receiver 296 forreceiving transmissions from the cable modem termination systemincluding line card 1042 via the upstream data flow and medium accesscontrol (Subscriber MAC) 90 for providing an interface between the bursttransmitter 294, the continuous receiver 296 and subscribercommunications equipment such as a PC 1048 (FIG. 2), a telephone, atelevision, etc.

The burst receiver 292, Headend MAC 60 and continuous transmitter 290 ofthe cable modem termination system including line card 1042 and theburst transmitter 294, Subscriber MAC 90 and continuous receiver 296 ofeach cable modem may each be defined by a single separate, integratedcircuit chip.

Referring now to FIG. 5A, the cable modem termination system includingline card 1042 of FIG. 2 is shown in further detail. The cable modemtermination system including line card 1042 is configured to receivesignals from and transmit signals to an optical fiber 79 of the hybridfiber coax (HFC) network 1010 (FIG. 2) via optical-to-coax stage 49,which is typically disposed externally with respect to the cable modemtermination system including line card 1042. The optical-to-coax stage49 provides an output to the 5-42 MHz RF input 84 via coaxial cable 54and similarly receives a signal from the RF upconverter 78 via coaxialcable 54.

The output of the RF input 84 is provided to splitter 57 of the cablemodem termination system including line card 1042, which separates the5-42 MHz RF input into N separate channels. Each of the N separatechannels is provided to a separate QPSK/16-QAM burst receiver channel85.

Each separate QPSK/16-QAM burst receiver channel 85 is in electricalcommunication with the headend MAC 60. The headend MAC 60 is inelectrical communication with backplane interface 62 which provides aninterface to ROM 73, RAM 68, CPU 66, and 100BASE-T Ethernet interface64.

The headend MAC, 60 provides clock and a data output to the downstreammodulator 72 which provides an output to amplifier 76 through surfaceacoustic wave (SAW) filter 74. Amplifier 76 provides an output to 44 MHzIF output, which in turn provides an output to the RF upconverter 78.

Each burst receiver 85 is configured so as to be capable of receivingboth QPSK (4-QAM) or 16-QAM signals. The QPSK signals provide 2 bits persymbol, wherein each bit has ±1 amplitude levels. The 16-QAM signalsprovide 4 bits per symbol, each bit having a ±1 or ±3 amplitude level.

However, the description and illustration of a burst receiver configuredto accommodate QPSK and 16-QAM inputs is by way of illustration only andnot by way of limitation. Those skilled in the art will appreciate thatother modulation techniques, such as 32-QAM, 64-QAM and 256-QAM mayalternatively be utilized.

The cable modem 12 in FIG. 2 is shown in detail in FIG. 5B within arectangle 258. The system shown in FIG. 5B includes a diplex filter 259.The systems shown in FIGS. 5A and 5B can be combined into a single blockdiagram by rotating FIG. 5B through an angle of 180° so that the diplexfilter 259 appears in inverted form at the right end and by thendisposing the sheets adjacent each other.

The signals from the diplex filter 259 in the range of 54-860 MHz passto an RF tuner 260 and then to a surface acoustic waver filter (SAW) 261which provides signals at a suitable frequency such as approximately 44MHz to an amplifier 262. The amplified signals pass to a 64/256-QAMdownstream receiver 263 with forward error correction (FEC). Automaticgain controls are provided from the receiver 263 to the tuner 260. Clockand data signals then pass from the receiver 263 to a medium accesscontroller (MAC) 264 which introduces signals through an interface 265to individual ones of a 10 Base-T transceiver 266, a CPU 267, a randomaccess memory (RAM) 268 and a read only memory (ROM) 269.

The signals from the individual ones of the 10 Base-T transceiver 266,the CPU 267, the RAM 268 and the ROM 269 pass through the interface 265to the medium access controller (MAC) 264. The signals from the MACcontroller 264 are then introduced to a QPSK-16 QAM upstream burstmodulator 270 with forward error correction. The signals from the burstmodulator 270 are provided to a low pass filter 271 which passes signalsin the range of 5-42 MHz when the system is used in North America. Thelow pass signals are then introduced to a power amplifier 272, theoutput from which is provided to the diplex filter 259. The gain in thepower amplifier 272 is regulated by the burst modulator 270.

In order to provide an enhanced understanding of the invention, certainterminology used in this application will now be defined. A “MAP” isprovided from the headend 10 to the subscriber modem 12. A MAP defines anext frame. A “frame” is a generic term to define a group or, a multiplenumber of slots.

FIGS. 7A and 7B are block diagrams showing at the subscriber cable modem12 the encrypting and decrypting system discussed herein. In FIG. 7A,data packets with encrypted data and control information are received atthe cable modem 12 from the headend 10 by the receiver 296 (also shownin FIG. 4). The control information may illustratively indicate theinformation provided in a request contention region 486, a CM txopportunity region 488 or a maintenance region 490, all shown in FIG.36. The data packets are then introduced to a downstream processor 342which parses the data and the control information and introduces theencrypted (parsed) data through a line 343 (FIGS. 7A and 7B) to adownstream decryptor 344 (FIG. 7B). The decrypted data is thenintroduced from a downstream (D/S) direct memory access (DMA) 306through a memory interface 308 in a DMA controller 312 to a first areain a static random access memory (SRAM) 314. The control informationalso passes through a DMA 391 and the memory interface 308 in the DMAcontroller 312 to a second area in the SRAM 314.

When data is to pass from the SRAM 314 to the headend 10, the decrypteddata and the control information are read from the separate areas in theSRAM and are passed through an upstream direct memory access (DMA) 522in the DMA controller 312. An upstream header processor 319 introducesdecrypted information from the SRAM 314 to an upstream header processor319. The decrypted data is then encrypted in an upstream data encryptionstandard (DES) circuit 321. The encrypted data from the DES 321 and thecontrol information from the upstream header processor 319 thenrespectively pass through lines 322 and 323 (FIGS. 7A and 7B) to anupstream control 324 in FIG. 7A.

The upstream control 324 provides an interface which receives timingfrom a timing regeneration circuit (TRC) 341 to control the time whenthe encrypted data passes from the DES circuit 321. The encrypted dataand the control information are then combined in the transmitter 325.(also shown as transmitter 294 in FIG. 4) at the subscriber modem 12 toform the extended packets. A serial peripheral interface) 326 providesan interface for control information between the upstream control 324and an SPI bus leading to a tuner and EEPROMS.

Each individual subscriber has an encryption unique to that subscriber.This encryption is encoded by the headend 10 in packets sent to thatindividual subscriber and is decoded by the individual subscriber. Inlike manner, the encryption is encoded by the subscriber modem 12 inextended packets sent by the individual subscriber to the headend 10 andis decoded by the headend.

FIGS. 8A and 8B are block diagrams similar to those shown in FIGS. 7Aand 7B. However, FIGS. 8A and 8B show the system at the headend 10 forencrypting the data in packets sent by the headend to the individualsubscriber modem 12 and for decrypting the packets sent by theindividual subscriber modem to the headend. The system shown in FIGS. 8Aand 8B may be disposed on an integrated circuit chip.

As shown in FIGS. 8A and 8B, packets of data may be introduced to theheadend 10 by a data queue 327 in a server external to the integratedcircuit chip or may be introduced to the memory from a local businterface 328 or a CPU interface in the chip. The interface 328 providesa control for a direct memory access (DMA) engine 329 similar inconstruction to the DMA 306 in FIG. 7B. The packets from the data queue327 are stored in a downstream (D/S) data buffer or FIFO 533. Thepackets are parsed by a downstream parser 364 and the data in the parsedpackets is encrypted by a DES encryption engine 535. The encryption isdifferent for each individual subscriber modem 12 and is controlled by aDRAM access controller 531, which accesses a key DRAM 728.

The encrypted data from the DES encryption engine 535 are introduced toa cyclic redundancy code/header check sum (CRC/HCS) inserter 361. TheCRC/HCS inserter 361 provides a parity check to make certain that thepacket is complete. The CRC/HCS inserter 361 combines the encrypted datafrom the DES encryption engine 535 and the other control informationfrom the downstream parser 364 in the extended packet. The extendedpackets meeting the tests of the CRC/HCS inserter 361 are passed througha line 389 in FIGS. 8A and 8B and are stored in a downstream transmitbuffer TxFIFO 390 in FIG. 8B. The stored information and timinginformation from a timing generation circuit 341 are introduced to adownstream controller 392, which arbitrates between data and timinginformation.

The signals from the downstream controller 392 pass through a downstream(D/S) data interface 396 to the transmitter 537 external to theintegrated circuit chip. The transmitter 537 transmits the packetsdownstream to the subscriber modem 12 identified by the encryption inthe packets. A serial peripheral interconnection provides an interfacefor control information, but not data, between the integrated circuitchip (FIG. 8A) and the transmitter 537 and between the integratedcircuit chip and a plurality (e.g., eight) of receivers 394, which areexternal to the integrated circuit chip.

An upstream (U/S) data interface 395 is connected between each ofreceivers 394 and a corresponding upstream receive (U/SRx) buffer memory(Rx FIFO) 555. The information in the buffer memory 555 is introduced toan upstream channel arbiter 397. The arbiter 397 selects the packetsfrom one of the eight receivers at each instant in accordance with thesource of the data provided in a MAP FIFO 274. For example, the packetsfrom one of the receivers 394 may be selected when the packets aremarked with the code for that receiver in the MAP for that channel.

The packets passing through the arbiter 397 are stored in the FIFO 523in FIG. 8A and are introduced from the FIFO 523 through a line 431 to anupstream parser 557. The parser 557 passes the data to a DES datadecryption engine 434 and the other (e.g., control) information to acyclic redundancy code/header check sum (CRC/HCS verification) stage444. The DES decryption engine 434 decrypts the encrypted data under thecontrol of the DRAM access controller 531 and passes the decrypted datato the CRC/HCS stage 444. The CRC/HCS stage 444 combines the decrypteddata and the other information to re-form the extended packets andpasses the reformed packets to a buffer or FIFO 445. The packets thenpass through the DMA engine 329 to the host system memory disposedexternally of the integrated circuit chip in a server.

A serial peripheral interface (SPI) controller 426 in FIG. 8Acorresponding to the serial peripheral interface (SPI) 326 in FIG. 7A isconnected to the local bus interface 328 to provide an interface forcontrol information to write into the data queue 327. A managementinformation base (MIB) 432 stores statistical errors produced by thestage relating to undetected data packets, uncorrectable data packetsand signal-to-noise ratios in data packets, for use in connection withFIGS. 32 and 33.

The burst receiver used to practice this invention is shown as the block292 in FIG. 4. As shown in FIG. 4, the burst receiver is disposed at theheadend 10 to receive packets of symbols from the subscriber modem 12.As illustrated in FIG. 35, Each packet includes a preamble 720, a uniqueword 721, an equalizer train 722, a payload 723, and a guard time 724.

The preamble may be limited to as few as 16 symbols. It includes a firstgroup of symbols which have a binary alternating sequence in aparticular pattern to provide for a fast synchronization of the headend10 to the carrier frequency of the signals from the subscriber modem 12.It may also include symbols which distinguish the subscriber modem 12from the other subscriber modems on the channel.

The unique word 721 is in a distinctive symbol pattern to indicate theend of the preamble 720 and the beginning of the payload 723. Thepayload 723 may be of variable size depending upon the length of thecommunication from the subscriber modem 12 to the headend 10. Theequalizer train 722 may be provided between the unique word 721 and thepayload 723. The equalizer train 722 may be in a random sequence. It isprovided during the initialization period to train the equalizer toprovide proper coefficients to the subscriber modem 12.

Additional details in the construction of the burst receiver 292 areshown in FIG. 9. In FIG. 9, incoming radio frequency (IN RF) signals areintroduced on a line 460 to a downconvert stage 514 which converts thesignals to an intermediate frequency. The signals then pass to ademodulator 448 which recovers the modulated data. The signals from thedata demodulator 448 are introduced to an equalizer 453 which mayillustratively be for constellations designated as 16-QAM.

The signals from the equalizer 453 are introduced to a preambleprocessing stage 520. The stage 520 processes the preamble 720 toprovide for a very fast synchronization of the headend 10 to thefrequency of the carrier signals from the subscriber modem 12. This isimportant in insuring that the headend 10 will process all of the datasymbols in the packets from the subscriber modem 12.

The stage 520 also provides a ranging operation on the symbolstransmitted from the subscriber modem 12 to the headend 10. One aspectof this ranging operation is to determine the time between thetransmission of the symbols from the headend 10 to the subscriber modem12 and the transmission of symbols from the headend to the subscriber inresponse to the symbols transmitted from the headend to the subscriber.

Since the distance between the headend 10 and the subscriber modem 12may be as great as approximately one hundred (100) miles, the timebetween the transmission of symbols from the headend 10 to thesubscriber modem 12 and the response of the subscriber to the headendmay be large. Until this time is determined and a window is provided atthe headend around this determined time, the headend 10 cannot operateeffectively in processing the symbols from the subscriber.

The ranging operation involves the determination at the burst receiverof such parameters as the ranging offset measurement, the equalizercoefficients, the burst power level, the slot timing error and thecarrier frequency offset. The signals from the stages 520 are introducedto the demodulator 448.

In addition to being introduced to the stages 520, the signals from theequalizer 453 are introduced to a de-randomizer 275. The de-randomizer275 de-interleaves the signals which have been previously interleaved atthe subscriber modem 12 to prevent data from the subscriber from beinglost as a result of noise in the cable. The de-randomized signals thenpass to a Reed-Solomon (RS) decoder 524 which corrects for errors in thepackets. The signals then pass through MAC 60 (also shown in FIG. 4) toan output line 526.

Reference is made to FIG. 10 for additional details of the burstreceiver shown in FIG. 9. These additional details include a fine mixer462 which processes the received quadrature phase signals on the lines538 and 540. The fine mixer 462 also receives signals from a directdigital frequency synthesizer (DDFS) 463 which is constructed in a wellknown manner to provide signals for mixing with the signals on the lines538 and 540 to provide beat frequency signals.

The quadrature phase signals from the fine mixer 462 respectively passthrough low pass filters 464 and 465 to a clock frequency recovery stage552. The clock frequency recovery stage may include a phase locked loopwith a numerically controlled oscillator to provide a fast recovery ofthe frequency of the carrier signals from the subscriber modem 12. Aphase locked loop with a numerically controlled oscillator may begenerally known in the prior art but not for the purpose of providing afast recovery of the frequency of the carrier signals from a subscribersuch as the subscriber modem 12.

The quadrature phase signals from the clock frequency recovery stage 552pass to decimation filters 554 and 466. The decimation filters 554 and466 change the frequency of the signals from the clock frequencyrecovery stage 552 to a suitable frequency such as four (4) times thesymbol rate. The signal then pass to Nyquist filters 558 and 467. TheNyquist filters 558 and 467 constitute matched filters which providesignals at the desired frequency.

The signals from the Nyquist filters 558 and 467 are in turn introducedto a clock phase recovery stage 468. The clock phase recovery stage 468may include a phase locked loop with a numerically controlled oscillatorto provide a recovery of the phase of the carrier signals from thesubscriber modem 12. A phase locked loop with a numerically controlledoscillator may be generally known in the prior art, but not for thepurpose of providing a fast phase recovery of the carrier signals from asubscriber such as the subscriber modem 12.

There are significant differences between the prior art and applicant'ssystem involving frequency and phase recovery of the carrier signalsfrom the subscriber 12. These differences cause applicant to recover thefrequency and phase of the carrier signals significantly faster than inthe systems of the prior art. Applicant's system provides separate clockfrequency recovery and clock phase recovery stages and disposes theNyquist filters between the clock frequency recovery, and clock phaserecovery stages. In the prior art, clock frequency recovery and clockphase recovery stages are combined into a single stage and the Nyquistfilters are disposed after this single stage.

A power estimator and start-of-burst detector stage 276 receives signalsfrom a stage 481 designated as “Ranging Process.” The ranging process isdescribed in detail below. The start-of-burst detector responds tostart-of-burst signals which are initially provided in the packet 719 inFIG. 35 at the headend 10 to indicate the time between the transmissionof symbols from the headend 10 to the subscriber modem 12 and thereception of return signals by the headend from the subscriber. Thesestart-of-burst signals are preferably start-of-burst signals in the samepattern as provided by the headend 10 to the subscriber modem 12 butthey may be in other patterns without departing from the scope of theinvention. As previously discussed, the distance between the headend 10and the subscriber modem 12 may be as great as one hundred (100) miles.This involves a total delay of approximately one and six tenthsmilliseconds (1.6 ms) between the transmission of signals from theheadend 10 to the subscriber modem 12 and the return of signals from thesubscriber to the headend.

FIG. 11 illustrates the delay between the transmission of signals fromthe headend 10 designated HE to the subscriber modem 12 designated SUand the return of signals from the subscriber to the headend. In FIG.11, time is indicated along the horizontal axis and distance along thevertical axis. The signal is shown as being transmitted from the headend10 at a time 482 as a downstream message to the subscriber modem 12. Thesubscriber modem 12 then processes the message during a time 484designated as “T process”. After processing the message, the subscribermodem 12 then sends an upstream message, which is received at theheadend 10 at a time 493. The transmission time between headend 10 andsubscriber modem 12 is designated Tpg. The contention resolutioninterval (CRI) is the sum of 2Tpg and “T process”. A ranging window 495is provided at the headend to indicate the time period during which theheadend 10 would ordinarily expect to receive the return signals fromthe subscriber modem 12. As shown, the duration Tdd of the window 495encompasses the time period “T process”. A time indication 499 is shownat the middle of the window 495 to indicate the time that the returnsignal from the subscriber modem 12 would ordinarily be expected at theheadend 10.

The start-of-burst signals initially transmitted from the headend 10 tothe subscriber modem 12 are in a simple binary pattern. Signals are thentransmitted by the subscriber modem 12 to the headend 10, preferable inthe same pattern as the start-of-burst signals transmitted from theheadend to the subscriber. In order for the headend 10 to act upon thesesignals, the signals have to be above a particular power level. Theyindicate to the headend 10 the that the subscriber modem 12 is going tobe sending, preferably immediately thereafter, to the headend 10 signalsfor initial maintenance. These initial maintenance signals are indicatedat 490 in FIG. 36.

Thereafter, the headend 10 sends maintenance signals periodically to thesubscriber 12. The time periods allocated by the headend to thesubscriber modem 12 for this subsequent maintenance can be quite precisebecause of the action of the time-of-burst signals in determining thetime between the transmission of signals from the headend 10 to thesubscriber modem 12 and the return of the signals from the subscriber tothe headend.

The signals from the stage 468 in FIG. 10 are introduced to a trackingloop 575 which provides a phase locked loop for the payload 723 in thepackets 719 in FIG. 35. The signals from the tracking loop pass to clockgenerator logic 501 as do the signals from a fast clock recovery circuit579. The tracking loop 575, the clock generator logic 501 and the fastclock recovery circuit 579 facilitate the operation of the clock phaserecovery stage 468 in recovering the phase of the carrier signals fromthe subscriber modem 12.

The signals from the tracking loop 575 also pass to a phase read onlymemory (ROM) 503. The memory 503 also receives signals from a fastcarrier recovery stage 581. The stage 581 may include a phase lockedloop for processing the preamble 720 in the packets 719 in FIG. 35 on afast basis to recover the frequency of the carrier signal. The phase ROM503 and the stage 581 are included in a derotator (FIGS. 21 and 22) thatprovides a carrier phase derotation of the signals from the fast carrierrecovery stage 581. Such derotators are well known in the art.

The quadrature phase signals from the stage 468 are respectively coupledby a carrier phase de-rotator stage 577 to a pair of multipliers 583 and509 and to an input terminal of an amplitude estimator 585. The signalsfrom the amplitude estimator 585 are also introduced to the multipliers583 and 509. The amplitude estimator interpolates the derotatedquadrature phase signals from the stage 581 to determine the peaks ofthese derotated signals. The amplitude estimator 585 then decimates theinterpolations between the peaks so that only the peaks remain.

The peak signals from the amplitude estimator 585 are then introduced toan equalizer 596 which operates in a well known manner to eliminate fromthe peak signals noise from extraneous sources and noise fromreflections in the line between the headend 10 and the subscriber modem12. Equalizers corresponding to the equalizer 596 are known in the art.The operation of the equalizer 596 is controlled by an equalizer control598.

The quadrature phase equalizer signals then pass to slicers 511 and 513,which are known in the art. The slicers 511 and 513 provide a pluralityof amplitude levels depending upon the constellation (e.g., 4-QAM,16-QAM) of the signals being processed and select the individual one ofthese amplitude levels closest in amplitude levels to the amplitudes ofthe peak signals from the amplitude estimator 585.

The outputs from the slicers 511 and 513 pass to a base band processor587 and to a unique word detector 515. The unique word (UW) detector 515detects the end of the preamble 720 and the beginning of the payload 723in the packets 719 in FIG. 35. The output from the unique word detector515 is introduced to the base band processor 587 to control theoperation of the processor. Among other functions, the base bandprocessor 587 provides forward error correction to correct errors in thepayload 723 in the packets 719 in a well known manner.

The burst receiver 292 shown in FIGS. 9 and 10 has certain importantadvantages. It provides for a determination by the headend 10 of thefrequency and phase of the carrier signals from the subscriber modem 12in as few as sixteen (16) symbols in the preamble 723 (FIG. 35), notcounting the start-of-burst signals initially included in the preamble.This is advantageous because it is important to acquire a fastacquisition of symbols in each packet 723, particularly since the burstreceiver 292 is acquiring data from different subscriber modem insuccessive regions in each frame. If the symbols in each packet are notacquired fast, valuable information may be lost.

The burst receiver 292 also provides for ranging functions (e.g., slottiming, power level, carrier frequency (FIG. 4) after the determinationof the timing interval, by the start-of-burst signals, between thetransmission of signals from the headend 10 to the subscriber modem 12and the return of the signals to the headend has been determined. Theburst receiver 292 shown in FIGS. 9 and 10 also provides for eachsubscriber to send and receive the signals in the packets 719 inprogrammable constellations (e.g., QPSK, 16-QAM) in accordance with thesignal-to-noise ratio in the line between the subscriber modem 12 andthe headend. It further provides for different subscriber modems in thesame channel to send and receive signals in the packets in differentconstellations and at different baud rates (e.g., 160 K Baud, 5.12 MBaud). This is true even within successive regions in the same frame.This occurs on a subscriber-by-subscriber basis for the differentsubscriber modems in the channel.

As previously described each channel includes a plurality of subscribermodems, each of which can operate with different constellations and atdifferent baud rates. Furthermore, as shown in FIG. 31, the subscribermodem in different channels can operate in a range of frequencies fromfive megahertz (5 MHz) to forty-two megahertz (42 MHz) in North Americaand at frequencies even higher than forty-two-megahertz (42 MHz) inforeign countries. This causes differences of power between the signalsin the subscriber modem in different channels to have a range as fiftydecibels (50 db). This is a considerable dynamic range in power. Itwould be accordingly difficult to compensate in a single stage for sucha considerable difference.

This invention provides a system for compensating in a simple andefficient manner for differences in power as much as fifty decibels (50db) between subscriber modems in different channels. The systemaccomplishes this by providing a portion of the compensation while thesignals are in analog form and by providing the remaining portion of thecompensation after the analog signals have been converted to a digitalform.

A system for accomplishing the objectives discussed in the previousparagraph is shown in FIGS. 25A and 25B. It includes a filter 610 havinga filtering range of approximately five megahertz (5 MHz) to forty-twomegahertz (42 MHz) corresponding to the frequency range of the channelsin North America. The signals from the filter 610 are introduced to ananalog amplifier 612.

The differences in the level of power in the filter 610 and the analogamplifier 612 for different subscriber modems 12 may be high because thesignals are provided through the entire range of frequencies in thechannels. (See FIG. 31 for the range of frequencies in the channels.) Asuitable portion of the dynamic range of power of approximately fiftydecibels (50 db) for the different subscriber modems 12 is handled inthe filter 610 and the analog amplifier 612. For example, this portionmay be approximately twenty decibels (20 db). This is accomplished bysetting the gain threshold of the analog stages so that power isobtained from the amplifier 612 only above a specified level representedby the threshold.

The signals from the filter 610 are also introduced to ananalog-to-digital (A/D) converter 614. The signals from the converter614 pass to a wide band power estimator 616. The output of the powerestimator 616 is introduced to a stage 618 for regulating gain. Theoutput from the stage 618 passes to an input to the analog amplifier612. The output of the analog amplifier 612 is connected to a burstdemodulator 519. The A/D converter 614, the wide band power estimator616, the gain regulating stage 618 and the burst demodulator 519 areincluded in the burst receiver 292 which is indicated in FIGS. 25A and25B by broken lines.

The power estimator 616 measures the power of the signals from thesubscriber modem 12 at each frequency in a channel. This can beaccomplished by shifting the frequency of the power estimator 616through the different frequencies in the channel. In this way, the powerestimator 616 measures the average power of the signals transmitted inthe channel by the subscriber modem 12 to the headend 10. The gainregulating stage 618 regulates the value of this average power at aparticular value. The regulated gain is introduced to the analogamplifier 612 which amplifies the signal from filter 610 and introducesthe amplified signal to the burst demodulator 519. In this way, theburst demodulator 519 handles the remaining portion of the dynamic rangeof power. This may illustratively be approximately thirty decibels (30db) when the dynamic range of power for the subscriber modem 12 in thedifferent channels is approximately fifty decibels (50 db).

When the burst receiver 292 receives one of the packets 719 from thesubscriber modem 12, it determines if the unique word 721 matches thepattern for the unique word at the burst receiver. If such a matchoccurs, this indicates that the applicable payload 723 follows in thepacket 719. A count is then made of a particular number of symbols fromthe end of the unique word in the direction toward the preamble. Thiscount may include a portion of the preamble. The amplitude of thesignals in each of these symbols is then determined and the average ofthese amplitudes in each of these symbols is then computed.

The average amplitudes for the different symbols are then added and thesum is divided by the number of symbols involved in the computation toobtain a resultant value. This value is inverted and the inverted valueis latched. The latched value is used to recover the payload 723 tooffset any difference between a desired amplitude and an actualamplitude for the bits in the payload symbols. For example, if theresultant value is 2. the inverted value is accordingly ½ and this valueis latched. So one-half (½) of the amplitude of each bit in each symbolis used as the amplitude value of the bit.

To determine a power value that is used for correction in FIG. 4, acount is made of a particular number of symbols from the beginning ofthe unique word in the direction toward to the preamble. This count mayinclude a portion of the preamble. The average power in each symbol inthe count is then determined and the average power in the differentsymbols is added to obtain a resultant value. This resultant value isdivided by the particular number of symbols to obtain the correctionalvalue that is used in obtaining the power level at the subscriber modem12 as shown in FIG. 4 and described above.

Referring now to FIG. 14, a contemporary, continuous transmission isshown wherein a series of contiguous payloads 1101, such as thosedefined by data packets, are concatenated to define a generallycontinuous data stream 1100. Because the data stream 1100 is generallycontinuous, e.g., does not contain periodic interruptions, acquisitiononly occurs infrequently, such as during startup or initialization.

The contemporary data stream 1100 as shown in FIG. 14 is suitable forpoint-to-point transmission, such as between a single transmitter and asingle receiver.

Referring now to FIG. 15, data bursts 11105 define a discontinuous datastream 1103. The data bursts 1105 are typically defined by data packetsand are separated by guard bands 1107.

The area between guard bands 1107 where the data bursts 1105 are locatedis defined by a time division multiple access (TDMA) time slot which thecable modem termination system including line card 1042 pre-assigns tocable modems 1046 which have previously requested such time slots inorder to facilitate upstream communications. The guard bands 1107provide some tolerance between adjacent time slots, so as to mitigatethe occurrence of undesirable data collisions between adjacent datapackets.

It is possible, such as in light data traffic conditions, that one ormore adjacent time slots might be empty, thereby further increasing thetime between adjacent data bursts.

Because the data bursts 1105 are discontinuous, each data packet whichdefines a data burst must be reacquired by the burst receiver 85 (FIG.5A).

The discontinuous nature of such time division multiple access (TDMA)upstream communications is thus due to the fact that a plurality ofdifferent cable modems are competing for upstream channel bandwidth.Since the upstream channel is divided into a plurality of time slots, soas to accommodate the plurality of cable modems transmitting in theupstream channel, it is difficult, if not impossible, to define asingle, continuous upstream data transmission.

Thus, the discontinuous nature of the upstream data communicationnecessitates the use of a burst receiver which is capable ofre-acquiring each individual data packet.

Referring now to FIG. 16, the data packet which defines each data burst1105 comprises a QPSK or QPSK-like preamble (i.e., a subset of 16-QAMconstellations) 1109 and a 16-QAM payload 1110. The QPSK preamble 1109is an order of sixteen symbols long. It is during this sixteen symbolQPSK preamble 1109 that acquisition by the burst receiver 85 takesplace. It is during acquisition that fractional symbol timingcorrection, carrier phase correction and fine amplitude correction aredetermined, so as to facilitate proper and reliable demodulation of the16-QAM payload 1110.

Referring now to FIG. 17, the QPSK preamble 1109 comprises a binarypattern 1111 (better shown in FIG. 24) and a unique word 1112. Thebinary pattern 1111 of the QPSK preamble 1109 is used in the recovery oracquisition of fractional symbol timing and carrier phase. The uniqueword is used in the recovery or acquisition of fine amplitude. Theunique word also optionally provides an identification of thetransmitting cable modem 1046.

Fractional symbol timing correction and carrier phase correction areboth determined by fast feedback loop processes. According to thepresent invention, fractional symbol timing correction is performed by afractional symbol timing phase locked loop and carrier phase correctionis performed by a carrier phase correction phase locked loop, both ofwhich are discussed in detail below.

Referring now to FIG. 18, a contemporary phase locked loop 1120comprises a phase detector 1122 to which a first signal is provided atinput 1124. The signal provided to the first input 1124 is a signalhaving some degree of timing information. The timing of the signalprovided to input 1124 may not be stable and/or may not be comprised ofwell-defined, substantially noiseless pulses, such as those of anoscillator or a clock.

When it is desired to provide a comparatively stable, well-definednoiseless reference signal, such as that which may be used to facilitatesampling in an analog-to-digital converter, it is necessary to use theoriginal signal to facilitate timing recovery.

The phase detector 1122 provides an output which is proportional to adifference in phase between the signal provided at input 1124 and afeedback signal provided at input 1125. Because the output of the phasedetector 1122 typically comprises an undesirable high frequencycomponent, loop filter 1127 is used to assure that only desirable lowfrequency components of the output of the phase detector 1122 areprovided to voltage controlled oscillator 1129. The output of voltagecontrolled oscillator 1129 is provided as a reference signal or thesecond input 1125 to phase detector 1122. The output of voltagecontrolled oscillator 1129 also forms the desired comparatively stable,well-defined, substantially noise-free reference for use in suchapplications as clocking or sampling.

Referring now to FIG. 19, a simplified phase locked loop 1140 atypically used in a digital receiver, which includes a matched filter1145 a in addition to the standard phase locked loop components of aloop filter 1148, a numerically controlled oscillator 1149 and phasedetector 1147, is shown. The phase locked loop shown in FIG. 19 is asomewhat simplified version of the fractional symbol timing correctionphase locked loop of FIG. 22, which likewise includes a matched filter.

It is important to understand that the matched filter 1145 a of thesimplified phase locked loop 1139 a inherently represents an undesirabletime delay, and thus, undesirably increases acquisition time of thephase locked loop 1139 a.

Referring now to FIG. 20, according to one aspect of the presentinvention (shown in detail in FIG. 22), the matched filter 1145 a ismoved outside of the phase locked loop 1140 b so as to remove theundesirable time delay from the loop and thereby improve the acquisitiontime thereof. The phase locked loop shown in FIG. 20 is a somewhatsimplified version of the fractional symbol timing correction phaselocked loop of FIG. 22, wherein the matched filter has been movedoutside of the phase locked loop.

Referring now to FIG. 21, according to one aspect of the presentinvention, a burst receiver circuit 1139 a comprises a fractional symboltiming phase locked loop 1140 a, a carrier phase correction phase lockedloop 1141 and an amplitude estimator circuit 1142. As shown in FIG. 21,the matched filter 1145 a of the fractional symbol timing phase lockedloop 1140 a is inside the phase locked loop defined by resampler 1146,phase detector 1147, loop filter 1148 and numerically controlledoscillator 1149. According to this aspect of the present invention, thematched filter 1145 a introduces an undesirable time delay, therebyinhibiting fast acquisition of the fractional symbol timing.

The burst receiver 1139 a shown in FIG. 21 includes an analog-to-digitalconverter 1150, a down converter which includes a direct digitalfrequency synthesizer 1151 and a mixer or multiplier 1152, and a lowpass filter 1153 for removing unwanted high frequency components whichresult from the mixing process of the down converter.

The carrier phase correction phase locked loop 1141 includes the phasederotator 1160, phase detector 1161, loop filter 1162 and numericallycontrolled oscillator 1163, which operate as discussed above withrespect to FIG. 9 so as to provide a phase reference signal to the phasederotator 1160 which corrects error in the carrier phase and smallresidual frequency error of the data packet which otherwise would tendto cause errors in the amplitude demodulation or slicing process.

Conventional coherent amplitude estimator circuit 1142 includes aconventional coherent amplitude estimator 1165 which operates accordingto well-known principles to provide an amplitude estimate or correctionfactor after fractional symbol timing and fine carrier frequencysynchronization have been achieved.

The amplitude estimate or correction factor is applied to the datapacket via multiplier 1166 before the data packet is input to equalizer1170 which compensates for channel spectral deficiencies which wouldotherwise inhibit reliable amplitude demodulation by the slicer 1171.

Referring now to FIG. 22, burst receiver 1139 b is identical to burstreceiver 1139 a with the exception that matched filter 1145 b is outsideof both phase locked loop 1140 b, so as to avoid the undesirableinherent introduction of a delay in acquisition time caused by placingthe matched filter 1145 b within the timing recovery loop 1140 a asshown in FIG. 21. This mitigates undesirable delays within thefractional symbol timing loop caused by the matched filter. The minimumdelay in the loop is essential for fast burst acquisition using thefeedback loop architecture. An open-loop resampler is also required toresample the input signal relative to the symbol rate, independent ofthe ADC sample clock rate. Since the matched filter 1145 b is placedahead of resampler 1146 of the fractional symbol timing phase lockedloop 1140 b, a second resampler 1154 must be provided ahead of matchedfilter 1145 b so as to provide the digitized data burst to the matchedfilter 1145 b at a proper sample phase of the data.

Referring now to FIG. 23, phase detector gain boosting logic mayoptionally be provided so as to further enhance the speed at whichacquisition occurs. As those skilled in the art will appreciate, theloop filter 1201 of a phase locked loop 1200 is sensitive to theamplitude of the signal provided thereto. That is, the coefficientsselected for the loop filter 1201 must be appropriate for the amplitudeof the signal provided to the loop filter 1201 in order to assure rapidacquisition of the signal input to the phase locked loop 1200.

In order to assure that the amplitude of the signal input to the loopfilter 1201 is within a desired range, i.e., is appropriate for thecoefficients selected for the loop filter 1201, a sensor and amplitudecontrol 1202 monitors the amplitude of the voltage input to phasedetector 1203 and modifies, via mixer or multiplier 1204, the amplitudeof the signal input to loop filter 1201 which provide an output to NCO1205. Thus, when the amplitude of the signal input to the phase detector1203 is too low, then the sensor and amplitude control 1202 increasesthe amplitude of the signal input to loop filter 1201 such that theamplitude of the signal input to loop filter 1201 is within a desiredrange which is appropriate for the coefficients thereof. Similarly, whenthe sensor and amplitude control 1202 senses that the amplitude of thesignal input to the phase detector 1203 is too high, then the sensor andamplitude control 1202 reduces the amplitude of the signal input to theloop filter 1201, such that the amplitude of the signal input to theloop filter 1201 is within the desired range for the coefficients of theloop filter 1201.

Referring now to FIG. 24, a timing recovery accelerator enhances thespeed at which the binary preamble is acquired by assuring that thesamples taken by the sampling circuit are taken at a time which assuresreliable detection of the amplitude of the binary signal of thepreamble.

According to contemporary practice, a single clock signal 1300 (waveformA) is used to clock the sample circuit such that the sample circuitsamples the binary preamble on the rising edge 1301 of the clock signal1300, for example. However, in those instances when the rising edge 1301occurs near the zero or transition point 1310 of the binary pattern 1111preamble 1109, then the amplitude of the binary pattern 1111 may notprovide high enough phase detector gain to facilitate reliable amplitudedetection thereof.

According to contemporary practice, when this occurs the clock for thesample circuit is shifted in phase until reliable detection of theamplitude of the binary preamble occurs. However, as those skilled inthe art will appreciate, such shifting of the phase of the clock for thesample circuit is undesirably time consuming and thus, undesirablyincreases the acquisition time of the binary preamble.

The present invention uses two offset symbol sampling clocks 1302 a and1302 b (waveform B). Both of the offset symbol sampling clocks 1302 aand 1302 b effect sampling by the sample circuit on the rising edges1303 a and 1303 b, thereof respectively. The two offset symbol samplingclocks 1302 a and 1302 b are out-of-phase with one another, such as by180 degrees. Therefore, at least one of the two offset symbol samplingclocks 1302 a and 1302 b must have a rising edge 1303 a, 1303 b whichoccurs when the amplitude of the binary preamble 1111 is near itsmaximum and can therefore be reliably detected.

According to the present invention, that offset symbol sampling clockwhich provides samples having the highest absolute value (to account forthe negative voltage peaks) at the beginning of each burst (FIG. 24) isselected to provide a clock for the sample circuit. As those skilled inthe art will appreciate, by utilizing two offset symbol sampling clocksand selecting that offset symbol sampling clock which provides thehighest absolute value, the need to vary the phase of the symbol clockfor the sampling circuit is eliminated and the speed at whichacquisition is performed is substantially enhanced.

Thus, the present invention includes a receiver architecture having tworesampler circuits. One is an open-loop resampler for the symbol clockfrequency resampling and the other is a closed-loop resampler for symbolclock phase acquisition.

It is worthwhile to point out that two different power estimationprocesses may be performed by the burst receiver. The burst receiver mayperform a narrow band power estimation and/or a wide band powerestimation.

The narrow band power estimation relates to a particular symbol rate andis utilized to measure the narrow band channel noise power level, as apart of the channel estimate which is used to determine channel quality.

The wide band power estimate relates to the overall upstream band,typically from 5-42 MHz and may be used to determine the analogfront-end gain setting. Generally, it is desired that the analogfront-end gain setting be configured to handle up to 50 dB of dynamicrange at the upstream cable plant.

Referring now to FIG. 25B, a wide band RMS power estimator includesanalog-to-digital converter 1350 which receives a wide band input. Theanalog-to-digital converter 1350 provides an output to absolute valueblock 1351 which takes the absolute value of the input thereto andprovides an output to leaky integrator 1352 which integrates the signaland provides an output to an enable gate 1353.

The wide band RMS power estimator provides a coarse estimation which isused for initial variable gain amplifier setting.

A narrow band RMS power estimator is typically included after theNyquist filters and provides an average power of the I and Q channels.According to an exemplary embodiment, the narrow band RMS powerestimator has a 2-byte output and a relatively large time constant, ofthe order of tens of thousands of symbols. Noise channel powerestimation in an idle channel for spectrum management typically has adeviation of approximately 2 to 3 dB.

Robust Techniques for Optimal Upstream Communication

A plurality of upstream channels are periodically monitored for at leastone parameter which is indicative of channel quality. A first modulationmethod is used for each upstream channel for which the monitoredparameter(s) indicate that channel quality is above a predeterminedthreshold value and a second modulation method is used for each upstreamchannel for which the monitored parameter(s) indicate that channelquality is below the predetermined threshold value. The first modulationmethod utilizes a larger constellation size than the second modulationmethod, such that a higher data rate is achieved when the channelquality is good enough to support the higher data rate.

Further, the present invention provides in another aspect a method andapparatus for optimizing the efficiency of upstream data communicationsby dividing an upstream spectrum into a plurality of upstream channels,wherein each upstream channel has a bandwidth of less than or equal toapproximately 0.5 MHz. The upstream channels are periodically monitoredfor at least one parameter which is indicative of the quality of eachmonitored upstream channel. Communications are moved from a used channelan unused channel when the monitored parameter(s) indicate that thequality of the used channel is below a predetermined threshold. In thismanner, noisy channels tend to be avoided and communications occur onhigher quality channels which are capable of supporting higher datarates. The present invention in another aspect provides a method andapparatus for tending to optimize upstream communication efficiencywherein a communications channel having upper and lower frequency boundsis defined in an attempt to determine an optimal bandwidth of thechannel given constraints imposed by the presence of adjacent channels,as well as constraints imposed by narrowband interference. Data rate isthus enhanced by varying the symbol rate of communications performed viathe channel in a near-continuous manner, i.e., by varying the upperand/or lower frequency bounds, so as to enhance the bandwidth andthereby enhance the efficiency with which the available frequencyspectrum is utilized.

Thus, according to the present invention, the data rate of the upstreamchannel tends to be optimized, so as to enhance the data communicationefficiency of the upstream channel.

The present invention provides enhanced upstream data rates by utilizinga constellation which is efficient in view of transmission mediumconditions, by providing fine frequency agility based upon channelquality monitoring so as to enhance the effectiveness with which thelimited bandwidth of the upstream channel is utilized, and by providingnearly continuous symbol rate switching so as to facilitate the usage ofan enhanced symbol rate which is compatible with line conditions.

The method and apparatus for communicating information from a pluralityof cable modems to a cable modem termination system are illustrated inFIGS. 27 and 29-32, which depict certain exemplary embodiments thereof.FIGS. 26 and 28 depict prior art contemporary communications circuitry.

Referring now to FIG. 26, according to contemporary practice a cablemodem termination system includes a plurality of demodulators 700 a-700n which receive modulated data which is input from a plurality of cablemodems via a common transmission medium. The demodulators 700 a-700 nprovide a demodulated data output for the frequency division multiplexed(FDM) upstream channels via which data is transmitted from the pluralityof cable modems to the cable modem termination system (CMTS). The cablemodems communicate with the cable modem termination system via timedivision multiple access (TDMA), wherein a plurality of cable modemscommunicate with each demodulator 700 a-700 n and wherein the cablemodems associated with each demodulator 700 a-700 n are distinguishedfrom those associated with a different demodulator via frequencydivision multiplexing (FDM).

Referring now to FIG. 27, according to the present invention monitoringcircuit 336 of the cable modem system 10 monitors the ability of eachFDM channel to reliably transmit data at a desired data rate. That is, aparameter which is indicative of channel quality is periodicallymonitored so as to determine the ability of the channel to facilitateupstream data communications. The channel monitoring function is, forexample, incorporated into each individual burst receiver, of whichthere are typically eight per cable modem termination system. Theaveraging and statistic gathering function is may be common to allchannels, and thus may reside separate from the individual burstreceivers.

As used herein, channel quality is defined as the ability of a channelto transmit data reliably thereon, such that higher quality channelstransmit data reliably at a higher data rate than lower qualitychannels.

When a quality of the channel, such as signal-to-noise (SNR) isdetermined to be above a predetermined threshold value, then a firstmodulation method is utilized for that upstream channel. When thequality of a channel is determined to be less than that of thepredetermined threshold value, then a second modulation method isutilized. The first modulation method is capable of providing a higherdata rate than the second modulation method. According to theillustrated embodiment of the present invention, first modulation methodhas a larger constellation size than the second modulation method.According to the exemplary embodiment of the present invention, thefirst modulation method encompasses 16-QAM and the second modulationmethod encompasses QPSK (4-QAM). Thus, according to the presentinvention, a constellation size is selected which is dependent upontransmission medium characteristics, such that the data rate ofcommunications on each channel tends to be enhanced.

Alternatively, more than two different modulation methods may beutilized. Thus, a plurality of different modulation methods, whereineach individual modulation method is generally better suited for adifferent range of channel quality, may be utilized. In this manner, theefficiency of data communications is yet further enhanced.

The modulation method utilized for each upstream channel is communicatedfrom the monitoring circuit 336 to each demodulator 700 a-700 n of thecable modem termination system and is also inserted into the downstreammessage flow such that the modulation method is communicated to eachcable modem 12, thereby facilitating modulation by each cable modem withthe desired modulation method. Thus, each cable modem 12 includes ademodulator 715 for demodulating downstream data transmissions from thecable modem termination system 10 and also includes a modulator 716 formodulating upstream data transmissions. A control circuit 717 of thecable modem 12 controls the modulation method utilized by the modulator716 and also controls the physical layer parameters such as forwarderror correcting gain and guard time.

According to one exemplary embodiment of the present invention, the stepof periodically monitoring a plurality of channels contemplatesperiodically monitoring a signal-to-noise ratio for each of themonitored channels. The signal-to-noise ratios are monitored over aplurality of separate communications bursts and an average of thesignal-to-noise ratios is formed from the individual measurements. Thisaverage is compared to the predetermined threshold value so as todetermine whether or not a change is to be made to the modulationmethod.

In an exemplary aspect of the invention, the predeterminedsignal-to-noise ratio threshold value is approximately 20 dB. Thus, ifthe signal-to-noise ratio is equal to or greater than 20 dB, then thefirst modulation method is utilized and when the signal-to-noise ratiois less than 20 dB, then the second modulation method is utilized.

In particular, the monitoring circuit 336 defines a portion of the cablemodem termination system. Alternatively, the monitoring circuit 336 maybe separate from the cable modem termination system, but might belocated generally proximate thereto, such that an accurate assessment ofeach channel's ability to transmit data may be performed.

According to another exemplary embodiment of the present invention, thestep of periodically monitoring a plurality of channels contemplatesperiodically monitoring channel noise power. Thus, channel noise powermay be monitored and compared to a predetermined threshold value so asto determine which modulation method is to be utilized. Channel noisemay be monitored in addition to signal-to-noise ratio (SNR) and/or anyother desired parameter which is indicative of channel quality, such aschannel statistics. Thus, any desired combination of parametersindicative of channel quality may be utilized according to the method ofthe present invention.

According to yet another exemplary embodiment of the present invention,the step of periodically monitoring a plurality of channels contemplatesperiodically monitoring channel statistics for each of the monitoredupstream channels. Examples of the statistics which may be monitored foreach upstream channel include the number of packets undetected, thenumber of packets with corrected errors, the number of packets withuncorrected errors, the number of forward error correction blocks withcorrected errors, and the number of forward error correction blocks withuncorrected errors. Combinations of these criteria and/or other desiredcriteria may similarly be monitored. Thus, it will be appreciated thatsuch channel statistics provide an indication of the quality of anupstream channel which may be utilized to determine which modulationmethod may be utilized to reliably and efficiently transmit data uponthat channel.

Optionally, at least one physical layer parameter of a channel may bechanged in response to a change in quality of the channel. For example,forward error correcting gain and/or the guard time associated with achannel may be changed in response to a change in the quality of thechannel. This change in the physical layer parameter may be either inaddition to or separate from any change in modulation methods.

Although periodic monitoring of one or more parameters indicative ofchannel quality is performed according to the exemplary embodiment ofthe present invention, those skilled in the art will appreciate thatcontinuous monitoring at such parameters may alternatively be utilized,if desired.

Referring now to FIG. 28, a prior art method for receiving modulateddata at a cable modem termination system from a transmission medium,such as a coaxial cable, and for converting that received modulated datainto digital data suitable for computer use includes demodulating themodulated data via an upstream burst receiver 333 and providing thedemodulated data to medium access control (MAC) 213. Medium accesscontrol (MAC) 13 controls access of the cable modem termination systemto the transmission medium and provides a digital data outputrepresentative of a message transmitted from a cable modem.

The upstream burst receiver is configured so as to be capable ofdemodulating both QPSK and 16-QAM modulation formats within a TDMAframe. According to contemporary methodology, the upstream burstreceiver 333 is configured so as to demodulate modulated data from thetransmission medium according to a single, predetermined modulationmethod. The predetermined modulation method must be selected such thatit provides reliable data transmission for a wide range of transmissionmedium conditions. Of course, this necessitates that a modulationtechnique which provides reliable data transmission even under the worstexpected transmission medium conditions must be utilized. As thoseskilled in the art will appreciate, such a modulation technique, QPSKfor example, does not provide the enhanced data rates which may bepossible when better medium conditions are present. That is, when only asingle modulation method is utilized, then data rate must typically besacrificed in order to provide the desired reliability.

Thus, in order to limit interruptions to upstream communications, cablemodem systems typically utilize an upstream modulation method which iscompatible with the lowest expected channel quality. However, as thoseskilled in the art will appreciate, such worst case modulation methods(QPSK, for example) are inherently inefficient at higher channelqualities. The modulation methods used for lower channel qualitiesprovide reduced bit rates, while the modulation methodology suitable forhigher channel qualities provide higher bit rates.

It is important to recognize that upstream data communications arecharacterized by a plurality of different time division multiplexedchannels, wherein each individual channel originates from a differentcable modem or a different group of cable modems. Because thetransmission path between the cable modem termination system and eachindividual cable modem is not identical (even though a common coaxialcable may be utilized along some portion of the path), variations inchannel quality occur. Thus, a wide variation in channel quality amongchannels, i.e., cable modems, is typical. These variations may occurbecause, for example, some of the cable modems and/or their links to thecommon coaxial cable are located proximate noise sources.

Referring now to FIG. 29, according to the present invention the cablemodem termination system includes a spectrum management/allocationcircuit 340 which at least periodically monitors the plurality ofupstream channels for at least one parameter which is indicative ofchannel quality. For example, spectrum management/allocation circuit 340may receive signal-to-noise ratio or channel power values from theupstream burst receiver 333. Alternatively, the spectrummanagement/allocation circuit 340 receives packet/FEC status from anupstream MAC/PHY channel statistics circuit 334. The upstream MAC/PHYchannel statistics circuit 334 receives the output of the upstream burstreceiver prior to the output of the upstream burst receiver beingprovided to the medium access control 213 and the upstream MAC/PHYchannel statistic circuit 334 calculates the packet/FEC statistics,which is then provided to spectrum management/allocation circuit 340.

Any desired combination of signal-to-noise ratio, channel power, packetstatistics and/or forward error correction statistics may be utilized asthe parameter which is indicative of channel quality.

Such monitoring of the signal-to-noise (SNR), channel power, and/orpacket/FEC statistics facilitates the determination of which modulationmethod, e.g., QPSK or 16-QAM, is to be utilized as long as the qualityof the channel is deemed to be sufficiently good to facilitate the useof such modulation methods. When the quality of the channel isinsufficient to facilitate the use of the smallest constellation size,i.e., QPSK, then the spectral allocation of the channel is changed, asdiscussed in detail below.

Optional averaging circuit 346 averages a plurality of signal-to-noise(SNR) or channel power measurements to compensate for short termfluctuations therein.

When a change in modulation method is indicated, then the new modulationmethod is transmitted from a switch circuit 345 of the spectrummanagement/allocation circuit 340 to the upstream burst receiver 333 andis also transmitted to the affected cable modem via downstream messageflow, as mentioned above.

When the quality of a channel is determined to be sufficiently poor(such that even QPSK will not provide reliable data transmission), thenthat channel may be moved to a different frequency allocation. When thisoccurs, the new upstream channel frequency is transmitted to theupstream burst receiver 333 and is also transmitted to the affectedcable modem via downstream message flow.

A bandwidth selection circuit 348 of the spectrum management/allocationcircuit 340 thus facilitates the implementation of fine frequencyagility and the switch 345. The bandwidth selection circuit 348determines the bandwidth of each downstream channel and the switch 345effects switching to the desired channel by the upstream burst receiver333. The upstream spectrum is divided into a plurality of upstreamchannels and wherein each channel is characterized as having a bandwidthwhich is less than or equal to 0.5 MHz.

According to the fine frequency agility aspect of the present invention,the spectrum management/allocation circuit 340 monitors the upstreamchannels for at least one parameter which is indicative of the qualityof each monitored upstream channel and moves communications from a usedchannel to an unused channel when the monitored parameter indicates thatthe quality of the used channel is below a predetermined thresholdvalue.

Thus, according to the present invention, 16-QAM is the baseline ordefault modulation method and QPSK is the fall-back modulation method,which is utilized only when channel quality is insufficient to supportupstream data transmission utilizing 16-QAM. Channel reallocation is thefall-back method used when channel quality is insufficient for the useof QPSK.

The use of 16-QAM enhances channel bandwidth efficiency by a factor of2, typically from approximately 1.6 bits/Hz to approximately 3.2bits/Hz, while providing approximately 25 percent excess bandwidth. Forexample, the use of 16-QAM provides up to 20.48 Mbps at 5.12 Mbaud.

However, it is important to appreciate that the detection of 16-QAM ismuch more difficult than the detection of QPSK, since the demodulationis amplitude sensitive, as well as phase sensitive when utilizing16-QAM, whereas demodulation is only phase sensitive when utilizingQPSK.

It is expected that the above discussed combination of variableconstellation size and fine frequency agility, i.e., dynamic channelallocation, will enhance channel bandwidth efficiency substantially.

It is important to understand that the upstream bandwidth in a hybridfiber/coaxial (HFC) network is a scarce resource. The bandwidth itselfis comparatively small (approximately 37 MHz as compared to the muchgreater bandwidth of approximately 814 MHz for the downstream band). Theupstream band is shared by a plurality of cable modems and may be sharedby other services, such as cable telephony, as well. Further, channelimpairment such as ingress noise make burst transmission difficult.

Thus, the use of spectrum management according to the present inventionfacilitates reconfiguration of radio frequency (RF) channels in a mannersuch that the RF channels are not impaired by ingress or the like andalso are not utilized by other services. When channel impairments suchas ingress noise do occur, the channel can be reconfigured or moved toan unaffected radio frequency or channel.

According to the present invention, on-going channel monitoring, basedupon the use of packet-based statistics and/or signal-to-noise ratio(SNR) and/or channel noise power is performed by the upstream receiveror cable modem termination system. By performing such monitoring at thecable modem termination system, spectrum analysis is provided at asingle location and the need for an expensive, external spectrummanagement unit is eliminated.

Examples of the types of statistical information which may be utilizedby the spectrum management/allocation circuit 340 to determine whetherchannel quality is sufficient to support 16-QAM, sufficient to supportQPSK, or channel quality is insufficient to support either 16-QAM orQPSK and the channel must therefore be moved to a different spectralallocation, e.g., frequency band are provided below:

Statistics Number of packets (total) Number of packets undetected (nounique word) Number of packets with corrected errors Number of packetswith uncorrectable errors Number of FEC blocks (total) Number of FECblocks with corrected errors Number of FEC blocks with uncorrectableerrors

Referring now to FIG. 30, upstream channel quality information can alsobe used to facilitate the changing of physical layer parameters in realtime. FIG. 30 shows Reed Solomon coding gain for various T's for 16-QAMwhere K=64 bytes. It is clear that as coding gain increases (increasingT), the probability of incurring a data transmission error P(e)decreases for any given signal-to-noise ratio.

The ability to change physical layer parameters in real time allows agiven channel to be optimized when the channel quality is not low enoughso as to require either a change in modulation method or to necessitatethat the channel be moved. Thus, according to the present invention,forward error correction (FEC) coding gain is increased and/or longerguard times are provided so as to facilitate reliable data transmissionon such channels. Of course, it is understood that as the number offorward error correcting parity bytes is increased, coding gain iscorrespondingly increased for a given size of the information bytes.

Referring now to FIG. 31, the upstream transmitter of each cable modemand the burst receiver of the cable modem termination system utilizefine frequency agility so as to enhance the overall data throughput ofthe upstream band. Fine frequency agility includes both channelreallocation (so as to avoid channels having poor quality) and thedefinition of channels with a fine frequency resolution (so as toenhance the efficiency with which the frequency spectrum is dividedamong channels).

Thus, according to the present invention, fine frequency agilityfacilitates the definition of channels in the upstream band with aresolution of a few Hz. According to the present invention, upstreamchannels are characteristically defined in increments of about 1.0 Hz.Such fine tuning capability is particularly beneficial in the lowfrequency portion of the upstream band, where narrowband ingress isfrequently present.

The use of such fine frequency agility facilitates the precisedefinition of upstream channels such that the usable upstream bandwidthis enhanced. That is, upstream channels can be defined such that thebandwidth of each upstream channel is as large as possible withoutincluding those portions of the upstream frequency spectrum whichinclude narrowband interference. Thus, such fine tuning of the availablespectrum mitigates waste due to unused, but otherwise good, i.e., notnoisy, bandwidth proximate narrowband ingress. Such waste inherentlyresults from the use of coarser spectrum division.

The ability to define upstream channels in this manner is substantiallydependent upon the resolution with which the channels may be defined.Thus, the, finer the resolution for defining the channels, the morereadily such channels may be defined in a manner which optimizes thebandwidth (by mitigating waste of the available spectrum) thereof whilestill excluding undesirable narrowband interference.

Further, the carrier frequency may be fine tuned so as to avoid suchinterference in a non-uniform fashion. That is, each individual channelin the upstream band need not have the same bandwidth. Rather,non-uniform bandwidths may be utilized so as to tend to optimize theoverall upstream data throughput.

It is important to note that the existing DOCSIS/IEEE specification onlyfacilitates the allocation of upstream symbol rates (which areproportional to the upstream channel bandwidths) according topowers-of-two. That is, each greater symbol rate is, according toDOCSIS/IEEE specifications, twice that of the preceding symbol rate.Thus, the DOCSIS/IEEE specifications do not facilitate enhancement ofoverall data throughput, as does the present invention. According to thepresent invention, symbol rates, e.g., bandwidths, may be varied in anearly continuous manner, e.g., in 1.0 Hz increments. The ability tochange symbol rates in such a near-continuous manner is a direct resultof such fine frequency agility, wherein the bandwidth of each upstreamchannel can be defined to a resolution of a few Hz.

The ability to vary the upstream symbol rate according to other thanpowers of two is particularly important where multiple data rates, suchas those frequently required by modem applications, are not themselvesdefined in powers of two.

Further, in the upstream band, where undesirable ingress in a channelfrequently inhibits uniform channelization, being able to vary the datarate in a generally arbitrary manner allows data carrier frequencies tobe positioned in between two narrowband interferences in a manner whichtends to optimize the bandwidth thereof, so as to enhance overall datathroughput of the upstream band.

An example of variable symbol rates between 100 kBaud to 5.12 Mbaudwhich are supported by the upstream channel is provided below:

Symbol Rate Channel Width QPSK Date Rate 16-QAM Date Rate (kysm/sec.)(kHz, α = 25%) (kbits/sec.) (kbits/sec.) 128 160 256 512 160 200 320 640256 320 512 1,024 320 400 640 1,280 512 640 1,024 2,048 640 800 1,2802,560 1,024 1,280 2,048 4,096 1,280 1,600 2,560 5,120 2,048 2,560 4,0968,192 2,560 3,200 5,120 10,240 4,096 5,120 8,192 16,384 5,120 6,40010,240 20,480

As further shown in FIG. 31, a bad radio frequency (RF) channel 701 ismoved to an unused portion 702 of the spectrum when the channel power,e.g., spectral density, of the bad channel exceeds a predeterminedthreshold level 703 or when any other monitored parameter indicates thatchannel quality is below a predetermined threshold. In this manner,channels are reallocated to portions of the spectrum having a higherquality and data throughput is enhanced.

The band, i.e., portion of the spectrum, to which such a channel ismoved is the best channel available at that time. In this manner,channelization tends to utilize the best available channels and the datarates supported by each such best available channel tend to bemaximized. Thus, the overall upstream data rate is substantiallyenhanced.

Alternatively, when a channel is moved away from a spectral locationwhere the measured parameter which is indicative of channel qualityindicates that the quality of the channel is below the predeterminedthreshold, then the new channel may be assigned by any desired method oreven may be assigned arbitrarily, as long as the quality of the spectrallocation to which the channel is moved is above the predeterminedthreshold.

Referring now to FIG. 32, dynamic channel allocation control flow startsat block 704. The process continues for a predetermined evaluation timeas shown by decision block 705, wherein a loop is incurred until thepredetermined evaluation time has been exceeded. The evaluation time isthat time during which upstream channels are monitored, so as todetermine whether or not they are suitable for continued and/or futureuse. The evaluation time may be determined empirically.

During the evaluation time, if the number of undetected packets exceedsa predetermined threshold, as shown by decision block 706, then thesignal-to-noise ratio is checked. If the signal-to-noise ratio is lessthan a predetermined threshold, then the symbol rate and constellationfor a new, unused upstream channel is determined as, shown in block 712and a channel reallocation message is sent to all cable modems in thefrequency channel as shown in block 714.

If the signal-to-noise ratio is not less than the threshold, then themodulation for the upstream channel is changed to QPSK as shown in block711.

Similarly, when the number of uncorrectable packets exceeds apredetermined threshold as shown in decision block 707, then thesignal-to-noise ratio is compared to a predetermined threshold as shownin decision block 710 and symbol rate and constellation for a newupstream channel is determined as shown in block 712 when thesignal-to-noise ratio is less than the predetermined threshold and thechannel modulation method is changed to QPSK when the signal-to-noiseratio is not greater than the predetermined threshold, as shown in block710.

When the signal-to-noise ratio is less than a predetermined threshold asshown in decision block 708, then the number of corrected packets ischecked. When the signal-to-noise ratio is not greater than thepredetermined threshold, then the process repeats. When the number ofcorrected packets is greater than a predetermined threshold as shown indecision block 709, then the signal-to-noise ratio is checked withrespect to the predetermined threshold as shown in decision block 710.When the number of corrected packets is not greater than thepredetermined threshold, as shown in decision block 709, then theprocess repeats.

A spectrum analyzer 713 may be used to define the next availablechannel. The next available channel is that unused channel which is bestsuited for upstream communications. The local spectrum analyzer 713 maydetermine which unused channel is best suited for next use by looking atthe signal, or the power thereof, which is present upon the usedchannel. Of course, that unused channel having the lowest signal orpower is most likely best suited for use next.

Referring now to FIG. 33, CMTS dynamic channel allocation control flowis shown. Dynamic channel allocation starts 660, then waits 661 whiletwo independent flow paths execute. According to the left flow path, adetermination is made if the channel being monitored is bad, as shown indecision block 662. If the channel is not bad, then the process returnsto the wait 661 state while right control path continues to execute. Ifthe channel is bad, then the constellation is set to QPSK as shown inblock 663 and ingress cancellation is applied on a per channel basis asshown in block 664. Intersymbol interference (ISI) mitigation is appliedon a per user basis as shown in block 665.

If the signal-to-noise ratio is greater than a predetermined QPSKthreshold, as indicated in decision block 666, then a finalconstellation is assigned as shown in block 671 and the left controlpath returns to the wait state 661.

If the signal-to-noise ratio is not greater than the QPSK threshold,then the next available channel is provided, as shown in decision block667. The spectrum manager 668 controls this function. As shown by block670, the next available channel is assigned as the new channel forupstream transmission. If no next channel is available, then a newsymbol rate is assigned as shown in block 669.

As shown in the right control path, after the wait state 661 is entered,then each channel is checked on a per user basis as shown in block 672.If each channel, on a per user basis, is not found to be bad, then thewait state 661 is re-entered. If a channel is found to be bad on a peruser basis, then ISI mitigation is employed on a per user basis as shownin block 673.

The channel quality evaluation criteria include the use of undetectedpackets as indicating a bad SNR, uncorrectable packets as indicating amarginal SNR and corrected packets as indicating an acceptable SNR. Theequalizer acquisition time is configured such that ISI equalization isperformed in less than 100 symbols and the ingress canceller iseffective in 100 to 1,000 symbols.

It is understood that the exemplary method and apparatus describedherein and shown in the drawings represents only presently illustrativeembodiments of the invention. Indeed, various modifications andadditions may be made to such embodiments without departing from thespirit and scope of the invention. For example, those skilled in the artwill appreciate that various other measures of channel quality may beutilized for determining which modulation method is to be utilized upona given channel and to determine whether or not the spectral allocationof the channel should be changed. For example, the reliability withwhich various different types of messages are received may be measuredso as to provide such an indication of channel quality. Further, variousdifferent modulation methods, other than QPSK and 16-QAM, may beutilized. For example, the present invention may be utilized with32-QAM, 64-QAM and 256-QAM. Thus, these and other modifications andadditions may be obvious to those skilled in the art and may beimplemented to adapt the present invention for use in a variety ofdifferent applications.

Cable Modem Termination System Upstream MAC/PHY Interface

Referring now to FIG. 34, the present invention generally includes aninterface between the physical layer burst receiver 332 and the MAC 213of a cable modem termination system 10 which is in communication with aplurality of cable modems 12. The interface between the burst receiver332 and the MAC 213 includes a data interface for communicating datafrom the burst receiver 332 to the MAC 213, an error informationinterface for communicating error information from the burst receiver332 to the MAC 213 and a slot timing and data type interface forcommunicating information from the MAC 213 to the burst receiver 332, asdescribed in detail.

Referring now to FIG. 36, the contents of a MAP protocol data unit (PDU)487 are shown. The MAP PDU 487, which is transmitted on the downstreamchannel by the cable modem termination system 10 (FIG. 27) to all of thecable modems 12 on a given frequency channel, contains the time slotallocations for at least some of the cable modems 12 which havepreviously sent a request to transmit one or more data packets to thecable modem termination system 10. When the channel bandwidth issufficient, in light of the number of such requests received by thecable modem termination system 10, then the cable modem terminationsystem 10 allocates a time slot for each such requesting cable modem 12.

Further, the MAP PDU 487 at least occasionally defines at least onerequest contention region 486 and generally also contains a plurality ofcable modem transmit opportunities 488 within the upstream channel 491.A maintenance region 490 may also be defined by the MAP PDU 487 withinthe upstream channel 491, as discussed in detail below.

The request contention region 486 includes at least one time area withinwhich the cable modems 12 transmit their requests to transmit datapackets to the cable modem termination system 10. Each of the cablemodem transmit opportunities 488 define a time slot within which adesignated cable modem 12 is permitted to transmit the data packet forwhich it previously sent a request to the cable modem termination system10.

Additionally, one or more optional transmit contention regions (notshown) may be provided wherein cable modems 12 may contend for theopportunity to transmit data therein. Such transmit contention regionsare provided when sufficient bandwidth is left over after the MAP PDU487 has allocated transmit opportunities 488 to all of those cablemodems 12 which have requested a time slot allocation. Thus, transmitcontention regions are generally provided when upstream data flow iscomparatively light.

The upstream channel 491, is divided into a plurality of time intervals110, each of which may optionally be further subdivided into a pluralityof sub-intervals 489. The upstream channel 491 is thus partitioned so asto facilitate the definition of time slots, such that each of aplurality of cable modems 12 may transmit data packets to the cablemodem termination system 10 without interfering with one another, e.g.,without having data collisions due to data packets being transmitted atthe same time.

Thus, the use of a MAP 487 PDU facilitates the definition of time slots92. Each time slot 92 may be used for any desired predetermined purpose,e.g., as a request contention region 486 or a transmit opportunity 488.Each time slot 92, as defined by a MAP PDU 487, includes a plurality oftime intervals 110 and may additionally comprise one or moresub-intervals 489 in addition to the interval(s) 110. The number ofintervals 110 and sub-intervals 489 contained within a time slot 92depends upon the contents of the MAP PDU 487 which defines the time slot92. The duration of each interval 110 and sub-interval 489 may bedefined as desired. Optionally, each sub-interval 489 is approximatelyequal to a media access control (MAC) timing interval. Each MAP PDU 487defines a frame and each frame defines a plurality of slots 92.

The beginning of each sub-interval 489 is aligned in time with thebeginning of each interval 110 and each interval 110 typically containsan integral number of sub-intervals 489.

Typically, the request contention region 486 and each cable modemtransmit opportunity 488 includes a plurality of integral time intervals110. However, the request contention region 486 and/or the cable modemtransmit opportunity 488 may alternatively include any desiredcombination of intervals 110 and sub-intervals 489.

Thus, according to the present invention, each request contention region486 may be utilized by a plurality of the cable modems 12 to request oneor more time slot allocations which facilitate the transmission of oneor more data packets during the subsequently allocated transmitopportunity 488 of cable modem 12.

Each data packet may contain only data, although an extended data packetmay be defined to include both data and a preamble. The preamble istypically stripped from an extended packet by the cable modemtermination system 10 and the data in the packet is then processed by acentral processing unit of the cable modem termination system 10.

The duration of the request contention region 486 is typically variable,such that it may be sized to accommodate the number of cable modems 12expected to request time slot allocations from the cable modemtermination system 10. The duration of the request contention region 486may thus be determined by the number of requests transmitted by cablemodems as based upon prior experience.

The allocation of time slots 92 defined by cable modem transmitopportunities 488 may optionally be defined, at least in part, on thebasis of priorities established by the cable modem termination system 10for different cable modems 12. For example, priorities may beestablished for individual cable modems 12 on the basis of an electionmade by the subscribers, which is typically dependent upon the type ofservice desired. Thus, a subscriber may elect to have either a premium(high priority) service or a regular (low priority) service.

Alternatively, priorities may be established by the cable modemtermination system 10 for the cable modems based upon size and number ofcable modem transmit opportunities 488 historically requested by thesubscribers. Thus, a cable modem that typically requires a large numb erof time intervals 110 may be defined as a high priority user, and thusgiven priority in the allocation of time slots within a cable modemtransmit opportunity 488, based upon the assumption that such largeusage is indicative of a continuing need for such priority, e.g., isindicative that the subscriber is utilizing cable television,pay-per-view or the like. Alternatively, the cable modem terminationsystem may assign such priorities based upon the type of service beingprovided to each cable modem. Thus, for example, when cable televisionor pay-per-view is being provided to a cable modem, then the priority ofthat cable modem may be increased, so as to assure uninterruptedviewing. The priority associated with each cable modem 12 may determineboth the size of time slots allocated thereto and the order in whichsuch allocations are performed. Those allocations performed earlier inthe allocation process are more likely to be completely filled thanthose allocations performed later in the allocation process. Indeed,allocations performed later in the allocation process may go unfilled,when the bandwidth of the channel is not sufficient to facilitateallocation of time slots for all requesting cable modems 12.

Time slots which define the maintenance region 490 are optionallyprovided in a MAP 487. Such maintenance regions 490 may be utilized, forexample, to facilitate the synchronization of the clocks of the cablemodems with the clock of the cable modem termination system. Suchsynchronization is necessary in order to assure that each cable modem 12transmits only within its allocated time slots, as defined by each cablemodem's transmit opportunity 488.

The request contention region 486, cable modem transmit opportunity 488,and maintenance region 490 typically begin at the beginning of aninterval 110 and end at the end of an interval 110. However, eachrequest contention region 486, cable modem transmit opportunity 488, andmaintenance region 490 may begin and end anywhere as desired.

Thus, according to the present invention, variable duration requestcontention regions 486, cable modem transmit opportunities 488 andmaintenance regions 490 are provided. Such variable duration requestcontention regions 486, transmit opportunities 488, and maintenanceregions 490 facilitate flexible operation of the cable modem system andenhance the efficiency of data communications on the cable modem systemby tending to mitigate wasted channel capacity.

The current MAP 170 is transmitted in the downstream channel 485 aftertransmission of a previous MAP 91 a and before any subsequent MAP 91 b.Data, such as data packets associated with web pages, e-mail, cabletelevision, pay-per-view television, digital telephony, etc. aretransmitted between adjacent MAPs 91 a, 170, 91 b. The contents of eachcable modem transmit opportunity 488 optionally includes data and apreamble. The data includes at least a portion of the data packet forwhich a request to transmit was sent to the cable modem terminationsystem 10. The preamble typically contains information representative ofthe identification of the cable modem 12 from which the data wastransmitted, as well as any other desired information.

The data and the preamble do not have to occupy the full time intervalof the cable transmit opportunity 488. Guard bands 209 (FIG. 68) areoptionally provided at the beginning and end of each slot, so as todecrease the precision with which time synchronization between the cablemodem termination system and each cable modem must be performed. Thus,by providing such guard bands, some leeway is provided in the transmittime during which each cable modem inserts its data packet into theupstream channel 191.

Referring now to FIG. 37, the interrelationship of the MAC frame 179 ofthe MAC layer and the frames 128, each of which contain at least one,generally a plurality, of data 122 and forward error correction (FEC)494 portions which, taken together, occupy an allocated time slot 92(FIG. 36) which defines a request contention region 486.

Each MAC frame 179 further includes, at the physical layer, PHY overheadportion 120. The PHY overhead portion 120 contains physical layeroverhead information.

Additional PHY overhead portion 126 generally follows the forward errorcorrection (FEC) portion 494 and optionally includes a guard band 209which reduces the accuracy with which synchronization of the cablemodems must be performed. That is, the guard band of the physicaloverhead portion 126 provides a tolerance, such that the cable modems 12do not have to transmit precisely within their allocated time slots 92.

Each MAC frame 179 includes at least one PHY overhead portion 120, onedata portion 122 and forward error correcting (FEC) portion 494.Optionally, each MAC frame 179 may include a plurality of data portions122 and corresponding forward error correcting (FEC) portions 494, ifdesired.

Referring now to FIG. 38, a block diagram shows how the cable modemtermination system 10 at headend 1012 processes the priorities ofrequests made by the cable modems 12 to send data to the cable modemtermination system 10. As indicated by block 130, the cable modemtermination system 10 sends an initial or current MAP 170 (FIG. 36)based upon data previously collected from the cable modems 12 during thesending of a MAP 91 a previous to the current MAP 170. At the same time,the cable modem termination system 10 sets the time for the MAP 91 bsubsequent to the current MAP 170. The time for the subsequent MAP 91 bis based upon the time for the sending of the current MAP 170 and is setfor a time after the sending of the current MAP.

As indicated by block 132, the cable modem termination system 10collects collision statistics for the subsequent MAP 91 b while theinitial or current MAP 170 is being processed. These statistics indicatecollisions in time between bursts from different cable modems 12. Whenthe current time becomes greater than the subsequent MAP time, asindicated in block 131, then the MAP building process begins asindicated in block 134.

When an upstream request arrives as represented by a block 135, therequests for sending data in the subsequent MAP 91 b are then processedto determine if the requests are high priority bandwidth requests (block136 or low priority bandwidth requests (block 137). If the request is ahigh priority bandwidth request, the request is placed in a highpriority bandwidth request queue as indicated by a block 138. If therequest is a low priority bandwidth request, the request is placed in alow priority bandwidth request queue as indicated at block 139.

When a request is placed in either a high priority bandwidth requestqueue 138 or a low priority bandwidth request queue 139, a portion ofthe MAC, depicted as a line 140, provides a sequence for collectingcollision statistics for the subsequent MAP 91 b. Such a sequence isalso provided when a request is processed and is found to be neither ahigh priority bandwidth request 136 nor a low priority bandwidth request137.

Referring now to FIGS. 39 and 40, the construction of a frame is shown.As shown in block 143, requests are made by the cable modems 12 in arequest contention region 486 (FIG. 36) of a first MAP for the grant orallocation by the cable modem termination system 10 to the subscribersof Information Elements (IE). An Information Element may be consideredto be the same as a region. A maintenance opportunity is optionallyprovided as shown at block 144. Such maintenance opportunities may, forexample, be used to synchronize the operation of the cable modem 12 withthe operation of the cable modem termination system 10. As previouslyindicated, this maintenance opportunity may be provided onlyperiodically.

A determination is then made at block 146 as to whether the highpriority request queue is empty. If the answer is No with respect to thehigh priority request queue, a determination is then made at block 147as to whether the frame length is less than a desired length. If theanswer is Yes, the request of the subscriber to transmit data is grantedand the frame length is incremented by the size of the data requested atblock 148.

If the high priority request queue is empty, a determination is made atblock 149 as to whether the low priority request queue is empty. If theanswer is No, a determination is made at block 154 as to whether theframe length will be less than the desired length. If the answer is Yeswith respect to the low priority request queue, the request of the cablemodem 12 to transmit data to the cable modem termination system 10 isgranted and the frame length is incremented by the size of the grant.This is indicated at block 156.

It may sometimes happen that the frame length will be at least equal tothe desired length when the request with respect to the high priorityrequest queue is introduced to the block 147. Under such circumstances,the request is not granted and a determination is then made as towhether the low priority request queue is empty. Similarly, if the framelength will be greater than the desired frame length when a request withrespect to the low priority request queue is made, the request is notgranted. An indication is accordingly provided on a line 157 when thehigh priority request queue and the low priority request queue are bothempty or when the frame length will be at least as great as the desiredlength.

When the high priority request queue and the low priority request queueare both empty or when the frame length will be at least as great as thedesired length upon the assumed grant of a request, a determination ismade, as at block 158 (FIG. 40) as to whether the request queues areempty. This constitutes an additional check to make sure that the queuesare empty. If the answer to such determination is No, this indicatesthat the frame length will be greater than the desired frame length uponthe assumed grant of a request. Under such circumstances, a grant of azero length is provided in the MAP 170 for each request in each queue.This zero length grant is provided so that the headend can notify thesubscriber that the request has not been granted but was received by theheadend. In effect, a zero length grant constitutes a deferral. Therequest was seen, i.e., not collided, but not granted yet. It will begranted in a subsequent MAP 91 b.

If a determination is made as at block 158 that the request queues areempty, a determination is then made at block 162 as to whether the framelength will be less than the desired frame length. If the answer is Yes,the frame is padded to the desired length with data from a contentiondata region 168 (FIG. 42) in the frame, as indicated at block 164. Thecontention data region 168 constitutes an area of reduced priority inthe frame. It provides for the transmission of data from the cablemodems 12 to the cable modem termination system 10 via available slotsin the frame where cable modems have not been previously assigned slotsby the cable modem termination system 10. The contention data regiondoes not require a grant by the cable modem termination system 10 of arequest from a cable modem 12 as in the request contention data region486 in FIG. 36. Since no grant from the cable modem termination system10 is required, the contention data region 168 in FIG. 42 providesfaster access to data for the subscriber than the request contentionregion 486. The contention data region 168 is described below inadditional detail in connection with FIGS. 42 and 43.

Available slots in a frame are those that have not been assigned on thebasis of requests from the cable modems 12. As indicated at block 185 inFIG. 40, the cable modem termination system 10 acknowledges to the cablemodem 12 that the cable modem termination system 10 has received datafrom the contention data region (FIG. 41) in the frame. The cable modemtermination system 10 provides this acknowledgment because the cablemodem 12 would not otherwise know that such data was not involved in adata collision and has, indeed, has been received from the contentiondata region 168.

Referring now to FIGS. 41 and 42, a block diagram of that portion of thecable modem termination system 10 which receives requests from the cablemodems 12 and which generates MAPs in response to those requests isshown.

The contention data region 168 in FIG. 42 is included in frame 179defined by a MAP 170 (FIG. 36). The frame 179 in FIG. 42 may include anumber of other regions. One region is indicated at 172 and isdesignated as contention requests region 486 in FIG. 36. It includesslots 181 designated as X in FIG. 42. In these slots 181, collisionsbetween request data from different cable modems 12 have occurred. Otherslots 183 in the contention request region 172 are designated as R.Valid uncollided request data is present in these slots. The contentionrequest region 172 also illustratively includes an empty slot 175. Noneof the subscribers 14 has made a request in this empty slot 175.

A cable modem transmit opportunity region 176 (corresponding to thecable modem transmit opportunity region 488 in FIG. 36) may also beprovided in the frame 179 adjacent the contention request area 172. Aspreviously indicated, individual cable modems 12 are assigned slots inthis area for data in accordance with their requests and with thepriorities given by the cable modem termination system 10 to theserequests. Optionally, the cable modem transmit opportunity region 176may be considered as having two sub-regions. In a sub-region 178, slotsare specified for individual subscribers on the basis of requests of ahigh priority. Slots are specified in an area 180 for individualsubscribers on the basis of requests of a low priority.

The frame 179 may optionally also include a maintenance region 182. Thiscorresponds to the maintenance region 490 in FIG. 36. As previouslydescribed, the region 182 provides for a time coordination in the clocksignals of the cable modem termination system 10 and the cable modems12. The frame 179 additionally may optionally include a region 184 inthe contention data region 168 where a collision has occurred. Validdata is provided in an area 186 in the frame where no collisionoccurred. A blank or empty area 188 may exist at the end of thecontention data region 168 where further data could be inserted, subjectto potential collisions. It will be appreciated that the differentregions in the frame 179, and the sequence of these different regions,are illustrative only and that different regions and different sequencesof regions may alternatively be provided.

The signals of the frame 179 from different cable modems 12 a, 12 b, 12c, 12 d, etc. (FIG. 42) are introduced in upstream data processingthrough upstream channel 191 (FIGS. 41 and 42) to a TDMA demultiplexer192 (FIG. 41) in the cable modem termination system 10. Afterdemultiplexing, data in from the cable modems 12 a, 12 b, 12 c, 12 d,etc. pass from the demultiplexer 192 to a data interface 194. Thesignals at the data interface 194 are processed in an Ethernet system(not shown) or the like. The operation of the MAP generator 198 iscontrolled by data requests from the individual cable modems 12 a, 12 b,12 c, 12 d, etc. and by collision information which is indicative of thecable modems 12 a, 12 b, 12 c, 12 d, etc. attempts to insert data in thecontention data region 168. Thus, for example, a large number ofcollisions may indicate a need for a larger contention request region172 in the subsequent MAP. Attempts to insert data in the contentiondata region 168 may, optionally, be utilized by the MAP generator 198 toincrease the priority of any cable modem unsuccessfully attempting totransmit such data. The MAPs generated by the MAP generator 198 passthrough the multiplexer 196 and are broadcast by the cable modemtermination system 10 to the cable modems 12 a, 12 b, 12 c, 12 d.

A sample MAP 195 generated by the MAP generator 198 is generallyindicated in FIG. 41. The MAP 195 includes a region 189 where therequests of the cable modems 12 for Information Elements (IE) withinwhich to transmit data are indicated. As previously indicated, anInformation Element (IE) may be considered to be the same as a region.The MAP 195 also includes a region 193 where the cable modem terminationsystem 10 has granted the requests of the subscribers for InformationElements to transmit data. The MAP 195 additionally includes acontention data region 208 where the cable modem termination system 10has given the cable modems 12 the opportunity to transmit data inavailable spaces or slots without specifying the open spaces or slotswhere such transmission is to take place. An acknowledgment region 210is also included in the MAP 195. In this region, the cable modemtermination system 10 acknowledges to the cable modem 12 that it hasreceived data from the subscribers in the available slots in thecontention data region 208. As discussed above, the cable modemtermination system 10 has to provide such acknowledgment because thecable modems 12 will not otherwise know that the cable modem terminationsystem 10 has received the data from the cable modems 12 in thecontention data region 208.

After the ranging process has been performed so as to adjust the powerlevel of at least one cable modem so as to normalize the power of areceived transmission at the cable modem termination system, adjust thecarrier frequency of the cable modem so as to enhance channelization inthe frequency domain, and adjust slot timing of a transmission from thecable modem so as to compensate for propagation delays, data packetstransmitted from the cable modem to the cable modem termination systemmay be acquired by the cable modem termination system.

As those skilled in the art will appreciate, contemporary cable modemtermination systems include a burst receiver, a continuous transmitterand a medium access control (MAC) for controlling access of an externaldevice, such as a computer, network, or other data serving device to thephysical layer of the cable modem termination system.

In a contemporary cable modem termination system, the burst receivermerely communicates demodulated received data packets to the mediumaccess control, which forwards the received data packets to an externaldevice. Further communications occur between the burst receiver and themedium access control which enhance communications between a cable modemand the cable modem termination system.

More particularly, information is communicated from the MAC to the burstreceiver which is representative of parameters of received time divisionmultiple access data. According to one aspect of the present invention,the information representative of parameters of the received timedivision multiple access data is used by the burst receiver tofacilitate processing of the received time division multiple accesssignal, as described in detail below.

According to another aspect of the present invention, time divisionmultiple access communications are enhanced by communicating informationfrom the burst receiver to the MAC. The information is representative oferror conditions related to an upstream channel, as described in detailbelow.

More particularly, slot timing information and/or data-type informationis communicated from the MAC to the burst receiver to facilitateprocessing of upstream data packets by the burst receiver. The slottiming information include information representative of a start timeand a stop time of time division multiple access time slots. Thus, theslot timing information may include either start time and stop time,start time and duration or stop time and duration.

The slot timing and data-type information for each slot include astation or service identifier (SID) value which identifies atransmitter, e.g., cable modem, which is transmitting a data packetwithin the slot, the time at which the slot began and an interval usagecode which defines a burst type of the data packet transmitted withinthe slot.

The data-type information include information representative of aQPSK/QAM modulation type which was used to modulate the upstream datapackets. For example, the data-type information identifies the upstreamdata packet as being modulated by QPSK or 16-QAM.

As mentioned above, the communication of slot timing information and/ordata-type information is performed after a ranging process, such thatthe power level, carrier frequency and slot timing of the receivedupstream data packets have been adjusted to desirable values.

According to the other aspect of the present invention, errorinformation is communicated from the burst receiver to the MAC toestimate the channel quality. Thus, the error information facilitatesspectrum allocation or channel assignments and also facilitates themaking of adjustments to forward error correction gain in upstream datatransmissions. Also, the communicated error information facilitateschanges in guard band widths in upstream data transmissions.

The error information communicated from the burst receiver to the MAC isrepresentative of forward error correction errors and/or packet errorstatistics. Information is, for example, transmitted from the burstreceiver to the MAC as a series of data bursts. Error information isalso, for example, transmitted from the burst receiver to the MAC as aseries of data bursts. The data bursts for both the data and the errorinformation may be, for example, transmitted at a serial clock rate ofthe burst receiver.

According to one exemplary embodiment of the present invention, theerror information is transmitted from the burst receiver to the MAC asprepended information. Thus, the present invention optionally includesthe prepending of control information to data which is sent from theburst receiver to the MAC. The prepended control information includechannel statistics.

The channel statistics of the prepended information include, forexample, FECOK, correctable FEC error, uncorrected FEC error, no uniqueword detected, collided packet, no energy, and packet length violation.

The MAC may determine additional statistics from the prepended channelstatistics. Such additional statistics include, for example, number ofslots, number of slots with power but no data, number of slots with baddata, number of good data slots, total number of FEC blocks, number ofFEC blocks with correctable errors, number of uncorrectable FEC blocks,number of requests received, number of collided requests, number ofcorrupted requests, number of packets received, number of collidedpackets, number of corrupted packets, number of ranging messagesreceived, number of collided ranging messages received and number ofcorrupted ranging messages. Referring now to FIG. 35, a sample datepacket 719 includes a QPSK-like portion 725 and a QPSK or 16-QAM portion726 comprising a payload 723. Guard times 724 are typically used betweenadjacent data packets. The QPSK or QPSK-like portion 725 includes apreamble 720, a unique word 721, and, optionally, an equalizationtraining or ranging portion 722, which facilitates ranging, as describedin detail above.

Referring now to FIG. 43, the upstream minislot and interval definitionis shown. According to one example of the present invention, a minislotclock (MSCLK) defines a plurality of minislots 415, wherein each newminislot occurs at the rising edge 416 of the minislot clock 100. Aplurality of minislots typically define each request interval 417,maintenance interval 418, and data interval 419 of a MAP 420.

The request interval 417 of the MAP 420 defines a time period duringwhich the plurality of cable modems may contend for the transmission ofa request to the cable modem termination system. The request is arequest to send a specified amount of data from the cable modem to thecable modem termination system.

The maintenance interval 418 of the MAP 420 is used to facilitatehousekeeping functions such as ranging, as discussed in detail above.The data interval 419 of the MAP 420 defines the time slot within whichdata is transferred from a particular cable modem to the cable modemtermination system. It is information about this data interval 419 whichis communicated from the MAC to the burst receiver according to oneaspect of the present invention. This data includes informationrepresentative of the start time and the duration of the data interval419. By communicating information representative of the start time andduration of the data interval 419 from the MAC to the burst receiver,the burst receiver is able to process incoming data packets moreefficiently. Since the burst receiver knows when to expect data packetsfrom individual cable modems, the burst receiver is able to easilyseparate the data packets from different cable modems from one anotherand to perform subsequent processing thereupon.

Many of the terms and abbreviations discussed below are explained and/ordefined in the Data-Over-Cable Service Interface Specifications (DOCSIS)Radio Frequency Interface Specification SP-RFI-I02-971008, which is theMultimedia Cable Network System (MCNS) specification for cable modemsystems, the contents of which are hereby incorporated by reference.

Referring now to FIG. 44, the MAP message 421 format prior to processingby the message filter of the MAC is shown. The MAP message 421 typicallycontains a MAC management header 422 which contains information whichfacilitates desired processing by the MAC. An upstream channel ID 423indicates which upstream frequency channel the MAP message is to beapplied to. The UCD count 424 matches the value of the configurationchange count of the UCD which describes the burst parameters which applyto the MAP.

The number of elements 425 provides an indication of the number ofelements transmitted in this map. The allocation start time 427indicates the effective start time from cable modem termination systeminitialization (in units of minislots) available for assignmentsaccording to this MAP. The acknowledgment (Ack) 428 indicates the latesttime, from cable modem termination system initialization (in units ofminislots) which has been processed in the upstream data communications.This time is used by the cable modems for collision detection purposes.

The ranging back-off start 429 is an initial back-off window for initialranging contention, expressed as a power of two. Values for the rangingback-off start range from 0 to 15, wherein the highest order bits mustbe unused and set to 0.

The ranging back-off end 430 is the final back-off window for initialranging contention, expressed as a power of two. Values for the rangingback-off end 430 range from 0 to 15, wherein the highest order bits mustbe unused and set to 0.

MAP information elements 433 define the time slots during whichindividual cable modems transmit on a particular upstream channel to thecable modem termination system. A plurality of intervals of the MAP,such as the first interval 435, the second interval 436, and the lastinterval 437, define the individual time slots.

Each interval 435-437 includes a station or service identifier (SID)value 439, which identifies the cable modem for which the interval (andtherefore the time slot defined thereby, applies. SID equals 0 definesthe end of the list of intervals, thus indicating that all intervalshave been defined. The interval usage code (IUC) 440 defines the burstparameters to be utilized during the specified timing slot. Such burstparameters include the modulation type, e.g., QPSK or 16-QAM. The offset441 indicates when, with respect to a common time reference, eachinterval begins. Offset equals 0 defines a beginning of the firstinterval.

Optionally, each MAP has a fixed length and format, such that unusedintervals 442 may occur after the last interval 437. Acknowledgment anddeferrals 443 optionally may be inserted into the list of intervals,generally after the end of list 438.

Referring now to FIG. 45, after the format of the MAP message has beenfiltered by the MAC for communication from the MAC to the burstreceiver, the MAP message includes an allocation start time 150 and aplurality of MAP information elements 151-153. Each MAP informationelement generally includes a service ID 155 which identifies the cablemodem for which the slot time of the MAP information applies, aninterval usage code 159 which identifies the burst type utilized by thecable modem during the time slot, and also identifies the slot time,typically in units of minislots.

Referring now to FIG. 46, the architecture of an exemplary sharedSRAM-based MAP interface of a MAC for eight upstream channels of anexemplary cable modem termination system is shown. A MAP message filter171 receives downstream MAC frames which contain MAPs having the messageformat shown in FIG. 44, as it is constructed prior to filtering. TheMAP message filter 171 filters each MAP message so as to provide afiltered MAP message having a format such as that shown in FIG. 45.

After filtering, the contents of the filtered MAP are stored in 64×9 MAPinternal FIFO 161 and the channel to which the MAP message applies isstored in 16×8 channel select FIFO 173. SRAM controller 163 thencontrols the storage of the MAP internal FIFO 161 contents and thechannel select FIFO 173 contents in an external 64K×16 SRAM 174 and thenthe SRAM controller 163 effects the transmission of the contents of theexternal 64K×16 SRAM 174 to the appropriate, e.g., proper channel, MAPcontrol interface 165-167. Each MAP control interface 165-167 providescontrol signals from the MAC to one of eight burst receivers of thecable modem termination system. The control signals include minislotclock (MSCLK), Map Valid (MapValid), Map Clock (MapClk) and Map Data(MapData).

Instead of having an offset field which is defined in the originalDOCSIS specification, two adjacent offsets are subtracted so as tocompute the actual length of the interval in units of minislots.

Referring now to FIG. 47, the interface between each MAP controlinterface 165-167 (shown in FIG. 46) includes four conductors whichprovide communication from the MAP control interface to the burstreceiver demodulator 177. It is important to appreciate that generallythere is a dedicated MAP control interface 165-167 for each of eightburst receiver demodulators 177. Thus, there is a dedicated pair of MAPcontrol interfaces 165-167 and burst receiver demodulators 177 for eachupstream frequency channel.

Thus, FIG. 47 shows the signals for one channel of the MAP controlinterface between the MAC and the burst demodulator. There are a totalof four signals MSCLK, MapValid, MapData, MapClk, and each of these foursignals are replicated for each upstream channel. MSLK provides theburst demodulator with a timing reference for minislots. According tothe present invention, each rising edge of the MSLK signal defines thebeginning of an upstream minislot. The MapValid, MapData and MapClk forma serial interface which transfers the 32 bit MAP information elementstored in memory to the demodulator of the burst receiver at theappropriate time.

As shown in FIGS. 46 and 47, minislot clock (MSCLK), Map Valid(MapValid), Map Data (MapData) and Map Clock (MapClk) signals arecommunicated from each MAP control interface 165-167 to each demodulator177.

Referring now to FIG. 48, timing control of the MAP which defines theupstream request interval 480, maintenance interval 418 and datainterval 483 is shown.

The minislot count 730 begins at the beginning of the request interval480 and continues until the end of the data interval 483. The minislotclock 100 provides for the timing of the request interval 480,maintenance interval 418 and data interval 483 as discussed above. A MapValid signal 190 transitions to a low state 731 when Map data 732 ispresent.

Map Data 732 may, for example, contain 32 bits of data which define astation or service identifier (SID) 155 (14 bits) which identifies thecable modem which is providing the upstream communication, an intervalusage code 159 (4 bits) which defines the modulation type utilized bythe cable modem, and a region length and minislots 169 (14 bits) whichdefines the length of the time slot which contains the data beingtransmitted from the cable modem to the cable modem termination system.

A Map Clock 733, typically running at a much higher rate than theminislot clock 100, defines the timing of the station or serviceidentifier (SID) 155, the interval usage code 159 and the region lengthin minislots 169.

The MAP control interface of the MAC derives the starting time for eachMAP interval from the allocation start time and the length of theprevious MAP interval. The MAP control interface of the MAC transfersthe MAP information element (IE) when its internal minislot count isless than one starting time of the MAP interval. A MAP informationelement (IE) is transferred, as shown by the MapValid signal going low,after the minislot count turns to N+2, and the starting time of the MAPinterval for the information element (IE) being transferred is minislotN+3. For the burst demodulator, the rising edge of the minislot clock(MSCLK) signal right after a rising edge of the MapValid signal definesthe beginning of the MAP interval for the MAP information element (IE)just received.

The time critical MAP interface includes two important concepts. First,the processing of the MAP message and conversion to a simplified formatwhich is acceptable to the PHY or demodulator of the burst receiver,second, the use of a set of control signals to communicate this MAPinformation to the demodulator of the burst receiver utilizing a4-signal interface.

Referring now to FIG. 49, inference of the receive now (Rx now) 220signal for a maintenance interval 418, having a length of 6 minislots,for example, is shown. The Receive now 220 signal is provided at thebeginning of a minislot 415 which is the first minislot of themaintenance interval (as shown in FIG. 49) and provides an indicationthat the maintenance interval has begun.

For maintenance intervals, including initial and station maintenance,and data intervals, including both short and long data intervals, theburst demodulator is configured to receive only one packet per interval.Thus, the beginning of the MAP interval represents the Receive nowsignal for the burst demodulator.

Referring now to FIG. 50, in a similar manner, a Receive now 220 signalis inferred for a MAP 420 and is issued at the beginning of the firstminislot of the data interval 419.

Referring now to FIG. 51, a plurality of Receive now signals 220 may beassociated with each request interval, since each request interval maycontain a plurality of requests, each request from a different cablemodem. It is assumed in thus example that each request message requirestwo minislots to transmit.

The multimedia cable network system (MCNS) Data-Over-Cable ServiceInterface Specifications (DOCSIS) radio frequency interfacespecification (SP-RFI-I02-971008) protocol specifies a time-divisionmultiple access (TDMA) protocol for the upstream transmission of datapackets from cable modems to a cable modem termination system. In orderto send data upstream, each cable modem must request a data slot largeenough to hold the desired data. The CMTS responds to such request fromthe cable modems with a logical message (MAP) which is broadcast to allof the cable modems on a particular frequency channel. The MAP messagespecifies the upstream framing structure, so as to provide individualtime slots within which each cable modem may transmit. The MAP specifieswhich cable modems may transmit, when they may transmit, and how, e.g.,using what modulation type, they may utilize to transmit. When theappropriate TDMA time slot arrives (in time) a cable modem sends a burstof data, e.g., a data packet, to the cable modem termination system.Each cable modem is typically identified by one or more station orservice identifiers (SID). Each TDMA time slot is typically an integernumber of minislots, wherein each minislot is an arbitrary timingreference provided by the medium access control (MAC). The MCNS protocolnegotiates sets of transmission parameters between the cable modems andthe cable modem termination system. The parameters define how data isformatted during upstream bursts from each cable modem to the cablemodem termination system. The DOCSIS protocol currently defines sixburst types which may be used in upstream communications. Each bursttype defines the modulation to be utilized during such upstreamcommunications. The burst type is constant during a particular window intime, e.g., a time slot, and the burst type is designated by an intervalusage code (IUC).

The MAP message specifies which SID or cable modem has control ofupstream communications on a particular frequency channel during eachTDMA time slot. The MAP message also specifies the time at which thetime slot begins and which interval usage code or burst type is to beused. The number of minislots allocated for a particular time slot isdetermined, for example, by taking the difference between the currentTDMA time slot and the next TDMA time slot.

Ranging in power, slot timing and carrier frequency, as described above,is important in this TDMA communication system. Power control isrequired in order to normalize receive power at the cable modemtermination system, so as to mitigate inter-channel interference.Controlling carrier frequency ensures proper channelization and thefrequency domain for upstream communications. Collisions between datapackets and the time domain are mitigated by adjusting slot timing toaccount for different propagation delays between the cable modemtermination system and each individual cable modem on a given frequencychannel.

Besides the upstream adjacent channel noise or interference sourceswhich effect power, time and carrier frequency offsets, the upstreamchannel is also affected by other channel impairments, such as radiofrequency interference (RFI) noise. In order to maintain adequatechannel quality, channel error characteristics are monitored over timeso that as the channel improves or degrades, usage may be adapted, asdiscussed above. High level channel adaptation algorithms use theseparameters to preempt upstream channel failure by increasing forwarderror correction (FEC) coding gain, changing guard times and/or changingfrequency. Since the upstream channel consist of many TDMApoint-to-point links, the channel parameters are derived from astatistical analysis of simple accumulated measures such as FEC errorand packet error statistics, as described above.

In order to support such channel quality maintenance features, the burstreceiver and the MAC, according to the present invention, communicateappropriate information.

According to the present invention, MAC data is broken into FEC blocks,each FEC block is encapsulated with 2 status bytes and 0 to 46 bytes ofprepended information, status byte fields are used to pass errorinformation and enable statistics calculation and prepended datacontains ranging offsets and indicates when ranging is required, asdiscussed in detail below.

According to the present invention, three individual interfaces betweenthe medium access control (MAC) layer and the physical layer (PHY) orburst receiver are provided. These three individual interfaces are ashared purpose interface, e.g., either an SPI or I2C, a time criticalserial interface for specifying TDMA burst information such as thestation or service identifier (SID) and the interval usage code (IUC),and a dedicated data interface which is used to pass raw data andin-band control messages, as discussed in detail below.

The shared general purpose interface, typically either a SPI or an I2C,is used to configure non-time critical parameters such as burstprofiles, configuration parameters and reading status.

The upstream data is transmitted from the EHY or burst receiver to theMAC using a dedicated 3-wire data interface. This interface includes aserial data line, a free-running serial clock and a burst validindicator. Since the upstream data is processed in blocks, a singleupstream transmission may be transmitted between the burst receiver andthe MAC as a series of bursts at the serial clock rate.

For request and request/data regions, the demodulator of the burstreceiver is expecting to receive multiple packets during the interval,and there will be multiple Receive now signals which are received by thedemodulator of the burst receiver.

As shown in FIG. 43, there is a request interval of 6 minislots (3 ofwhich are represented) and it is assumed that each upstream requestmessage will need 2 minislots in order to transmit. Therefore, therewill be a total of three Rx now signals perceived by the demodulator ofthe burst receiver. These Rx now signals are located with offsets of 0,2 and 4 minislots from the beginning of the interval.

Referring now to FIG. 52, it is important to note that if the firstblock of a TDMA transmission bit is set, then the prepended informationincludes 2 status bytes, 4 timestamp bytes, 1 channel ID byte, 2 SIDbytes, 2 power bytes, 2 frequency bytes and 3 time bytes. The powerbytes, frequency bytes, and time bytes include a total of 7 bytesutilized for ranging offsets.

Referring now to FIG. 53, if the equalizer prepend bit is set, then theprepended information is increased by 32 bytes to provide a total lengthof 48 bytes, include 2 status bytes, 4 timestamp bytes, 1 channel IDbyte, 2 SID bytes, 2 power bytes, 2 frequency bytes, 3 time bytes and 32equalizer coefficient bytes. Again, the power bytes, frequency bytes andtime bytes define 7 bytes utilized for ranging offsets.

Referring now to FIG. 54, statistics are kept using counters as shown.These statistics are based upon bits [7:5] of the status bytes.

There are two important concepts with respect to the design of the MAPcontrol interface of the MAC. The first important concept regards theprocessing of downstream MAP messages, wherein conversion from a formatwhich is specified for the MCNS Data-Over-Cable Service InterfaceSpecification (DOCSIS) to a simplified format which is easy for the MACto process. The second concept regards the set of control signals whichare communicated between the MAP interface and the burst demodulator,including how the signals are toggled so as to convey to the burstdemodulator information such as when to receive a packet, the service ID(SID) associated with an incoming packet, the expected length of theregion where the packet is going to show up (the time slot for the datapacket), and the packet type.

Upstream bandwidth is divided into minislots, which are the smallesttime unit utilized by the MAP for bandwidth requests and grants. Theexact number of bytes per minislot is typically variable and is usuallyprogrammed into the MAC and the burst demodulator via a generic CPUinterface or the like. In order to define the minislot reference for theburst demodulator, the MAC provides a signal called the minislot clock(MSCLK) to the burst demodulator. Each rising edge of the minislot clocksignal defines the beginning of a new minislot as shown in FIG. 43.

An upstream interval generally consists of an integer number ofminislots. There are a plurality of different types, e.g., six differenttypes, of intervals currently defined by the MCNS DOCSIS specification,which include a request interval, an initial maintenance interval, astation maintenance interval, a short data interval, a long datainterval, and a request/data interval, as discussed in detail below. Therelationship of the request interval, maintenance interval and short andlong data intervals with respect to their minislots and the minislotclock is shown in FIGS. 43 and 48-50.

As discussed above, the MAP messages contain the information whichenables the burst demodulator to perform the task of receiving andseparating upstream packets. A message filter module is designed tosnoop all downstream packets and filter out the MAP messages containedtherein. The format of the MAP messages is simplified after suchfiltering. FIG. 44 shows MAP message format prior to MAP messagefiltering and FIG. 45 shows MAP message format after MAP messagefiltering, as discussed in detail below.

By encapsulating each MAC/PHY block with a 2 byte header, the PHY orburst receiver can pass in-band control information to the MAC. The MACcan then use the prepended information to collect channel statistics, aswell as pass link related information to higher processes, such asranging required. This in-band control allows the headend to senseimpending channel failure before such failure actually happens, therebyavoiding catastrophic data loss.

Referring now to FIG. 55, the upstream MAC/PHY interface, between theheadend (HE) or cable modem termination system (CMTS) MAC 213 and thedemodulator 34 of the burst receiver is shown. As discussed above, thisinterface includes a serial data interface 320, an SPI (or I2C)interface 330 and a serial control interface 240. The serial datainterface 320 facilitates the communication of upstream data, includingprepended information, from the burst demodulator to the MAC. The SPIinterface 330 is utilized for general configuration of the MAC and/orburst demodulator. The serial control interface 240 is a time criticalinterface utilized for the transmission of time critical MAP informationfrom the MAC to the burst demodulator.

According to the present invention, MAC data is broken into forwarderror correction (FEC) blocks. Each FEC block is encapsulated with 2status bytes and 0 to 46 bytes of prepended information. Status bytefields are used to pass error information and to enable statisticscalculation. The prepended data contains ranging, offsets and optionalequalizer coefficients. The upstream data is transmitted from the PHY(burst demodulator 34) to the MAC 213 using a dedicated 3-wire serialdata interface 320. The dedicated serial data interface includes aserial data line, a free-running serial clock and a burst validindicator. Since the upstream data is processed in blocks, a singleupstream transmission may be transmitted between the demodulator of theburst receiver and the MAC as a series of data bursts at the clock rate.In addition to the serial data, the MAC needs additional informationincluding identification as to which MAC/PHY bursts to which TDMA slots,as well as other information indicating error quality of the receivedTDMA transmission.

Referring now to FIG. 56, the free-running bit clock (BITCLK) 316, theburst valid indicator (BLKDV) 317, and the serial data line 302 havingprepended information 318 and data 304 thereon, are shown.

Referring now to FIG. 58, the format of the prepended data withequalizer coefficients is shown. The prepended data includes 2 bytes ofstatus flags 250, 4 bytes of timestamp 252, a 1 byte channel ID 254, a 2byte station or service identifier (SID) 255, 7 bytes of ranging offset256 and 32 bytes of equalizer coefficients 257. Using this prependedinformation, error conditions of the packet may be determined and thePHY parameters may be passed to higher level processes. Channelcondition statistics may also be maintained.

Referring now to FIG. 59, the subscriber PHY interface includes twoimportant concepts. First, control information is prepended to theactual packet data 360. This prepended information includes a burst-typebyte 362 and a packet length 363, generally of 2 bytes. The subscriberre-programs the PHY for each individual burst. Since this re-programmingmay require the exchange of significant amounts of data and must be donein real time, a particular architecture has been developed. Anon-real-time general purpose interface is used to program burst typeswhich are identified by a short ID (burst type). Only the ID and lengthare transferred in real-time in order to effect immediate re-programmingwithout being impacted by the speed of external interfaces. Real-timecontrol data is piggybacked on the transmit data interface so as toreduce complexity.

Referring now to FIG. 60, an initialization process is generallyutilized wherein a plug-and-play-like sign-on or registration sequenceis utilized between the headend or cable modem termination system 10which includes the burst receiver 332 and MAC 213 (FIG. 34) and thesubscriber or cable modem 12. According to this initialization process,a timebase message 398, a default configuration message 402 and asign-on message 403 are communicated from the, cable modem terminationsystem 10 to the cable modem 12 during initialization. A defaultconfiguration message (ranging channel frequency, transmission rate,initial power level, contention-based access slot information, etc.) issent for each downloaded frame and the timebase message and defaultconfiguration message facilitate upstream time configuration.

A sign-on message 403 transmitted from the cable modem terminationsystem 10 to the cable modem 12 facilitates contention based rangingperformed upon a dedicated channel. Typically, the cable modem 10responds with a sign-on response message 404. Then ranging 405 isinitiated by the headend 10 to determine slot timing corrections,carrier frequency corrections, and power corrections. Rangingcalibration responses 406 are transmitted from the cable modem 12 to theheadend 10.

Service channel, logic address and encryption key information aretransferred between the cable modem termination system 10 and the cablemodem 12 via re-provision message 407 transmitted from the cable modemtermination system 10 to the cable modem 12 and via re-provisionresponse message 408 transmitted from the cable modem 12 to the cablemodem termination system 10. When the process is complete, aninitialization complete message 409 is transmitted from the cable modemtermination system 10 to the cable modem 12.

Referring now to FIG. 61, the MAC framing and the PHY or burst receiverframing are decoupled and upstream frame synchronization is based ontimestamp messages (msgs). The cable modem termination system 10generates a timestamp message which is utilized by a subscriber cablemodem 12 to effect timing synchronization such that proper slot timingis facilitated. The output from a headend timing generation circuit 449is reduced in frequency by a divider 450 and is used by a timestampcounter 451 to generate slot/frame timing 452. The slot/frame timing istransmitted via continuous modulator 470 through analog front end 471over a desired downstream frequency channel 472, typically utilizing ahybrid fiber coax (HFC) network to the analog front end 473 of a desiredsubscriber cable modem 12. A continuous demodulator 474 demodulates theslot/frame time and provides it to a timing recovery circuit 475 whichutilizes a timestamp detector 476 to provide the slot/frame timing to adigital timing loop defined by loop filter 477, numerically controlledoscillator 478 and local timestamp counter 479. The loop generatesslot/frame timing for use by the subscriber cable modem 12 in generatingupstream TDMA messages.

Thus, the subscriber cable modem 12 is capable of transmitting upstreamdata via burst modulator 458 and analog front end 457 in a desiredupstream channel 456, typically via a hybrid fiber coax (HFC) network tothe analog front end 455 of the cable modem termination system, whereinthe message is demodulated by burst demodulator 34.

Referring now to FIG. 57, the MAP serial interface (MAC to PHY) fielddefinition includes a service ID 496 of 14 bytes, a slot type 497 of 4bytes and a slot region length 498 of 14 bytes.

Referring now to FIG. 62, the prepended information (form PHY to MAC)includes status information 735 of 2 bytes, minislot control 736 signalof 4 bytes, a channel ID 737 of 1 byte, a station or service identifier(SID) 738 of 2 bytes, ranging information 739 of 7 bytes and equalizercoefficients 740 of 32 bytes.

Referring now to FIG. 63, each MAC/PHY burst is tagged with 2 statusbytes as indicated.

Referring now to FIG. 64, burst demodulator status informationprocessing flow is shown. Forward error correction (FEC) 741 anddemodulation information 745 are added to prepended information 742, andreceiver TDMA control 744 is provided by MAP interface 743. The forwarderror correction 741 information includes forward error correctionstatistics such as: no forward error correction errors (No FEC ERR),correctable forward error corrected errors (Corr FEC Err), anduncorrectable forward error correction errors (Uncorr FEC Err).

The MAP interface information 743 includes service slot typeinformation. The receiver TDMA control 744 includes informationindicative of the first and last block of information. The demodulatorinformation 745 includes unique word detected, ranging information andequalizer coefficients.

Referring now to FIG. 65, the burst demodulator SPI bus interface isshown. The SPI interface 330 provides set-up information toconfiguration registers 560. The set-up configuration includes thecarrier frequency, the baud rate, the minislot size, the required packetsize, and ranging thresholds.

The SPI interface 330 also provides burst configuration information toburst configuration registers 562 via multiplexer 563 and data registers564, 565 and 566. The burst configuration information includes uniqueword pattern, unique word window, QAM mode, forward error correctionparameters, such as N, K and T, guard time, de-randomizer information,differential encoding information, preamble length and equalizertraining length.

Referring now to FIG. 66 (a detailed drawing of FIG. 59), the genericbyte base serial input interface at the upstream MAC/PHY interface atthe subscriber cable modem with control information prepended andTXS2P=1, includes the standard ATM signals of transmit clock (TX CLK)601 a, transmit enable bar (TX ENAB) 602 a, transmit cell available(TX_CLAV) 603 a, transmit start-of-cell (TX_SOC) 604 a, and transmitdata (TX_DATA) 605 a. When the control bit, TXS2P is set to equal 1,then transmitted data (TX DATA) 605 a is transmitted most significantbit (MSB) first.

Referring now to FIG. 67 (another detailed drawing of FIG. 59), thetransmit clock (TX CLK) 601 b, transmit enable bar (TX ENAB) 602 b,transmit cell available (TX_CLAV) 603 b, transmit_start-of-cell (TX_SOC)604 b and transmit data (TX_DATA) 605 b are shown when TXS2P is set tozero. When TxS2P is set to 0, then the least significant bit (LSB) istransmitted first.

Although the present invention is described and illustrated herein asproviding acquisition in 16 symbols or less, the present invention mayalso be utilized to provide acquisition in greater than 16 symbols. Forexample, the present invention may be utilized to provide acquisition in24 symbols. Thus, use of the present to provide acquisition in 16symbols or less is by way of example only and not by way of limitation.

Data Packet Fragmentation in a Cable Modem System

Data packets are transmitted from the cable modems to the cable modemtermination system within time slots which are allocated by the cablemodem termination system and wherein a data packet is fragmented ordivided among a plurality of time slots when a time slot which issufficiently large to contain the data packet cannot be defined due todata flow and bandwidth constraints.

Referring now to FIG. 68, the fragmentation of a data packet by a cablemodem into first and second portions thereof is shown, wherein the firstportion of the data packet is placed in a first time slot allocated bythe cable modem termination system and the second portion of the datapacket is placed in a second time slot allocated by the cable modemtermination system.

As shown in FIG. 68, a data packet 410 which is too large to fit withina first time slot 491 a is fragmented by an appropriate device such as acable modem such that a first portion 410 a of the data packet 410 isplaced in the first time slot 491 a and the remaining or second portion410 b of the data packet 410 is placed within the second time slot 491 bof the upstream channel 491.

For example, if the maximum size of each time slot 491 a, 491 b is 256symbols and the data packet 410 conforms 300 symbols, then 256 symbolsof the data packet 410 are put into the first time slot 491 a and theremaining 44 symbols are put into the second time slot 491 b.

This placing of the data packet 410, whether fragmented or not, withinthee allocated time slots 491 a and 491 b prevents undesirablecollisions among a plurality of such data packets transmitted by acorresponding plurality of cable modems upon a given frequency channel.The use of guard bands 209 in the upstream channel 491 tend to furthermitigate the occurrence of such undesirable collisions by providing anunused time space between each time slot of the upstream channel 491 soas to accommodate differences in synchronization between the cable modemtermination system and the various cable modems.

The assignment of such time slots is accomplished by providing a requestcontention area in the upstream data path within which the cable modemsare permitted to contend in order to place a message which requestsadditional time in the upstream data path for the transmission of theirmessage. The cable modem termination system responds to these requestsby assigning time slots to the cable modems making such a request, sothat as many of the cable modems as possible may transmit their messagesto the cable modem termination system utilizing TDMA and so that thetransmissions are performed without undesirable collisions. This timeslot assignment by the cable modem termination system is known as agrant because the cable modem termination system is granting aparticular cable modem permission to use a specific period of time inthe upstream.

The cable modem termination system usually tries to match the grant tothe request so that the cable modem is given sufficient bandwidth forits transmission.

It is not always possible for the cable modem termination system toallocate a sufficiently large time slot in response to a request fromthe cable modem so as to contain all of the data packet for which therequest was sent. This insufficiently large time slot allocation isreferred to as a partial grant. This may happen, for example, whenupstream traffic between the cable modems and the cable modemtermination system is heavy. Thus, in such instances, it is desirable todivide or fragment the data packet among a plurality of such time slots.Another example is when the cable modem termination system supportsconstant bit rate services, such as voice, in the upstream direction.These services require grants at periodic intervals. When supportingthese types of services, the cable modem termination system may need tosend a partial grant to one modem in order to schedule a constant bitrate service for another modem.

It is desirable to define a system for fragmenting data packets whichminimizes wasted bandwidth. In accordance with the present invention, atechnique is provided for fragmenting data packets in a cable modemsystem wherein data packets larger than an allocated time slot are splitamong a plurality of time slots.

In response to receiving the request, the cable modem termination systemallocates a time slot for transmission of at least a portion of the datapacket from the cable modem to the cable modem termination system.Alternatively, the allocation may be performed by a different device,e.g., a device other than the cable modem termination system, referredto herein generally as a dynamic time slot controller.

Information representative of the time slot is transmitted from adynamic time slot controller, such as the cable modem terminationsystem, to the cable modem.

At least a portion of the data packet is transmitted from the cablemodem to the cable modem termination system within the allocated timeslot. Transmitting at least a portion of the data packet from the cablemodem to the cable modem termination system within the time slotmitigates undesirable collisions between data packets which aretransmitted by different cable modems to the cable modem terminationsystem upon a common frequency channel. Thus, the simultaneoustransmission of data packets by different cable modems upon a commonfrequency channel is prevented.

When the time slot allocated for the transmission of the data packetfrom the cable modem to the cable modem termination system is sufficientfor transmission of only a portion of the data packet for which therequest was transmitted, then the cable modem transmits only a portionof the data packet for which the request was transmitted and the cablemodem termination system allocates at least one additional time slot andtransmits to the cable modem information representative of theadditional time slot(s), so as to facilitate transmission of theremaining portion of the data packet from the cable modem to the cablemodem transmission system.

Occasionally, due to bandwidth constraints and the amount of data flowon a given channel, it is not possible to allocate a time slot which issufficient for transmission of the entire data packet for which arequest was received by the cable modem termination system. Rather thandenying the request altogether, according to the present invention, atime slot is allocated by the dynamic time slot controller so as tofacilitate the transmission of at least a portion of the data packetfrom the cable modem to the cable modem termination system. One or moreadditional time slots are then allocated to facilitate transmission ofthe remaining portion of the data packet from the cable modem to thecable modem termination system. Thus, the remaining portion of thepacket may be split or fragmented among a plurality of such additionaltime slots, if necessary.

Briefly, upstream data transmission on an upstream channel is initiatedby a request made by a cable modem for a quantity of bandwidth, i.e., aplurality of time slots, to transmit data comprising a message. The sizeof the request includes payload, i.e., the data being transmitted, andoverhead, such as preamble, FEC bits, guard band, etc. After the requestis received at the headend, the CMTS grants bandwidth to the requestingcable modem and transmits the size of the grant and the specific timeslots to which the data is assigned for insertion to the requestingcable modem. If the grant is smaller than the size of the request, i.e.,a partial grant, the cable modem senses this condition and separatesdata into two or more fragments for transmission. If the cable modem hasnot received an additional grant or grant pending prior to thetransmission time of the partial grant, the cable modem inserts arequest for additional bandwidth into the fragment header. Thisbandwidth request, called a piggyback request, is for the amount ofbandwidth required to send the remainder of the packet. If the cablemodem has received an additional grant or grant pending prior to thetransmission time of the partial grant, the cable modem assigns a valueof zero to the piggyback request field in the fragment header. The cablemodem transmits the first fragment in the assigned time slots. The cablemodem treats each subsequent grant in the same manner. If additionalgrants (or grant pendings) are enqueued at the cable modem during thetransmission time of a fragment, the cable modem does not include apiggyback request. If additional grants are not enqueued at the cablemodem during the transmission time of a packet or fragment, the cablemodem inserts a piggyback request for enough bandwidth to transmit theremainder of the packet being fragmented. The piggyback field of thelast fragment of a packet can be used to transmit a request for theamount of bandwidth necessary to transmit the next packet enqueued atthe cable modem.

The CMTS can operate in either of two different modes: multiple grantmode or piggyback mode. In multiple grant mode, the CMTS must retain thestate of fragmentation for each modem. The CMTS allots bandwidth to therequesting cable modem and determines the amount of data required tofill the allotted bandwidth, taking into account the overhead requiredto transmit the fragment, and sends a partial grant for the fragmenteddata to the cable modem. The CMTS also transmits to the cable modem apartial grant for the remaining data fragment if there is bandwidthavailable in the current MAP or a grant pending if bandwidth must beprovided by a subsequent MAP. A grant pending signal is sent in eachsubsequent MAP until the grant can be fulfilled. In this mode, the cablemodems insert fragmented data pursuant to the grants as determined bythe CMTS.

In multiple grant mode, the CMTS does not need to retain the state offragmentation for each modem. The CMTS allots bandwidth to therequesting cable modem and the requesting cable modem determines theamount of data required to fill the allotted bandwidth, taking intoaccount the overhead required to transmit the fragment. The requestingcable modem inserts such data in the assigned time slots and checks forpartial grants or pending grants from the CMTS. If there are none, thisis a signal to the cable modem that the piggyback mode should be used.The requesting cable modem inserts a request for the remainder of thedata, including the amount of remaining data, in a piggyback field ofthe fragment header transmitted to the CMTS. (In the multiple requestmode, the piggyback field that accompanies a transmitted data fragmentis set to zero.) Responsive to the request in the piggyback field theCMTS transmits another grant to transmit data to the requesting cablemodem. If the alloted time slots are insufficient to transmit the entiredata fragment, the process is repeated until all the data has beentransmitted. In the piggyback mode, the cable modem retains the state offragmentation, i.e., it keeps track of the remainder of the data to betransmitted during the fragmentation process.

In summary, the cable modem is capable of operating either in themultiple grant mode or the piggyback mode, depending on how the CMTSallocates grants. If the CMTS generates partial grants or pendinggrants, this is sensed by the cable modem and the cable modem operatesin the multiple grant mode. If the CMTS does not generate partial grantsor pending grants, this is sensed by the cable modem and the cable modemoperates in the piggyback mode.

The size of the payload that can be transmitted in a specified number oftime slots depends on the burden imposed by the data transmissionformat. This size is called the burdened PHY length. In one embodiment,the burdened PHY length is determined by a forward lookup table usingthe total length of the data in bytes as an index. Each time a requestis made by a cable modem, the forward lookup table is accessed using thetotal length and the burdened PHY length is retrieved for transmissionto the CMTS as the request. Grants are transmitted to the cable modemsin terms of burdened PHY length. The total length of the data in bytesthat can be transmitted pursuant to a grant is determined by a reverselookup table using the burdened PHY length as an index. The forward andreverse lookup tables are created each time that the burst profilechanges and are stored in memory for use in processing requests andgrants at the cable modem. Alternatively, the conversion between totallength and burdened PHY length could be carried out as described inapplication No. 60/489,998 filed on Jan. 15, 1998.

Referring now to FIGS. 69 to 76, different aspects of an improved systemin which the cable modems 12 and the cable modem termination system 10cooperate to fragment packets of the data transmitted from the cablemodems 12 to the cable modem termination system 10 are shown. FIG. 69specifically shows a complete one of the extended packets 118. Theextended packet 118 is indicated in a block form at 118 a to showschematically the length of the extended packet. Details of the extendedpacket are indicated at 118 b in FIG. 69. As shown, the extended packet118 b includes a header portion 505. The header portion 505 may befurther defined by fields constituting a frame control (FC) 507, afragmentation MAC Header (PARM) 504 and a total length (LEN) 516 of theextended packet. The functions of the fields 507, 504 and 516 and thespecific implementation of these fields in binary coding are shown inFIG. 73.

The extended packet 118 b includes an extended length field or segment(EHDR) 517 which indicates the length of the data in the extended datapacket and which provides for the performance of a number of additionalfunctions shown in FIG. 74. For example, the field 517 may include flagsto indicate the first and last fragments in the packet 118 b when thepacket is fragmented.

The extended packet 118 b may also include a MAC Header Check Sequence(HCS) 518 which consists of 2 bytes and which insures the integrity ofthe sequence in the header in a known manner. The header check sequence518 is followed by data (PDU) 512 (the complete payload) in the extendedpacket 118 b. The data 512 is shown in cross-hatched lines in FIG. 69. Acyclic redundancy check (CRC) 521 follows the data 512. A cyclicredundancy check such as 521 is known in the prior art to provide anadditional check for insuring that the information in the packet 118 bis complete.

FIG. 70 is a schematic diagram of a concatenation of a number ofcomplete extended packets (such as that of FIG. 69) provided by one ofthe cable modems 12. The concatenation is indicated by a concatenationheader 525 which is followed by the information for the first of theextended packets in the concatenation. This information includes aheader (MAC HDR1) 527 for the first one of the concatenated packets, apayload (PDU) 528 for the first one of the concatenated packets and acyclic redundancy check 529 for the first one of the concatenatedpackets. Similar information is provided for the successive ones of theextended packets in the concatenation. The last one (the nth) of theextended packets in the concatenation is indicated by a MAC header (MACHDRn) 530, a payload (PDUn) 532 and a cycle redundancy check (CRCn) 534.The payload 528 and 532 are indicated in cross-hatched lines.

FIG. 71 is a schematic diagram of a plurality of data packet fragmentstransmitted from the cable modem 12 to the cable modem terminationsystem 10, wherein the data packet fragments form, in composite, acomplete data packet. FIG. 71 schematically shows different fragmentaryportions, generally indicated at 540 a, 540 b and 540 c, of a completepacket such as the complete packet 118 b in FIG. 69. Each of thefragmentation portions, 540 b and 540 c includes a fragmentation header542, a fragmentation header check sequence 544, a payload fragment 546and a fragmentation check redundancy cycle 548. Thus, the fragmentaryportions 540 a, 540 b and 540 c form, in composite, the complete payloadfor the packet 118.

Each of the fragmentation headers 542 includes a frame control (FC) 541,a segment (EHDR LEN) 543 indicating the length of the fragmentationheader 542, a length segment (LEN) 545 indicating the length of thefragmentation header 542 and an extended header (EHDR) 547 containingadditional information about the fragment.

FIG. 72 shows the fragmentary portion 540 a in additional detail. Asshown in FIG. 72, the fragmentary portion 540 a includes the framecontrol (1 byte) 541, a MAC_PARM (1 byte) 578, the indication (LEN) (2bytes) 545, the extended header (EHDR)(6 bytes) 547, the header checksequence (FHCS) (2 bytes) 544, the payload fragment 546 and the fragmentcyclic redundancy check (FCRC) (4 bytes) 548. The MAC_PARM 578 indicatesthe length of the EHDR 547.

The frame control 541 is shown in FIG. 72 as including an FC type 549.The FC type 549 indicates a media access controller (MAC) specificheader. The FC PARM 568 provides a fragmentation MAC header. The EHDR_ON550 indicates that a fragmentation EHDR follows. LEN indicates the totallength of the fragment including the payload, EHDR and FCRC.

The EHDR portion 547 may be considered as including an extendedheader-type (EH-Type) segment 572 which indicates the type of data inthe fragmentary portion 540 a. For example, the type of information inthe fragmentary portion 540 a may constitute unencrypted fragmentationdata or encrypted fragmentation data. Another segment in the EHDRportion is indicated at 574 and is designated as EH_LEN. It indicatesthe length of the EHDR. An additional segment 576 is designated asEH_Value. It provides different types of information. For example, itincludes a binary bit (or flag) which is set to a binary 1 to indicate afirst fragment for the payload in the extended packet 118 b and anotherbit (or flag) which is set to a binary 1 to indicate the last fragmentfor the payload in the extended packet. These bits are provided with abinary 0 for intermediate data fragments between the first data fragmentand the last data fragment in the extended packet 118 b. These flags forthe fragments shown in FIG. 71 would be F=1, L=0 for 540 a, F=0, L=0 for540 b, and F=0, L=1 for 540 c. The segment 576 also includes a sequencenumber which is incremented for each fragment of a packet. This sequencenumber can be set to zero for the first fragment within a packet orcontinue counting from the last fragment of the last packet transmittedfor this modem. This sequence number is used by the cable modemtermination system to detect lost fragments during packet reassembly.

FIGS. 73 and 74 provide a table indicating, in a first column, thedifferent types of fields shown in FIG. 72. FIGS. 73 and 74 also includea second column designated as Usage. This column indicates the differentsub-fields (if any) shown in FIGS. 69-72 for the different fieldsspecified in the first column of FIGS. 73 and 74 and specifies theoperations in such sub-fields. The number of bits in the sub-fields isindicated in a third column in FIGS. 73 and 74. The number of bytesspecified for each of the fields in the first column of FIG. 74 is shownin the fourth column of FIG. 74.

FIGS. 75 and 76 define a flowchart, generally indicated at 600, in blockform and show how the cable modem 12 and the cable modem terminationsystem 10 cooperate in the fragmentation of the payload 118 a forpackets transmitted by the cable modem 12 to the cable modem terminationsystem 10. The operation of the blocks in the flowchart 600 is initiatedat a start block 602. As indicated at block 604 in FIG. 75, the cablemodem 12 then awaits a packet from an external source. For example, theexternal source may be a personal computer (PC) 1048 (FIG. 2) at thehome 14 (FIG. 78) of a subscriber. As shown in block 606, the cablemodem 12 then submits to the cable modem termination system 10 abandwidth request for enough time slots to transmit the packet. Uponreceipt of the requests, the cable modem termination system sends agrant or partial grant to the cable modem in the MAP.

The cable modem 12 then checks as at block 609 to determine if the cablemodem termination system 10 has granted the request, or any portion therequest, from the cable modem 12. In block 609, SID is an abbreviationof Service Identification. If the answer is Yes (see line 607 in FIGS.75 and 76), the cable modem 12 then determines if the cable modemtermination system 10 has granted the full request from the cable modem12 for the bandwidth. This corresponds to the transmission of thecomplete data packet from the cable modem 12 to the cable modemtermination system 10. This is indicated at block 625 in FIG. 76.

If the answer is Yes, as indicated at block 625 in FIG. 76, the cablemodem 12 determines if there is another packet in a queue which isprovided to store other packets awaiting transmission to the cable modemtermination system 10 from the cable modem 12. This determination ismade at block 629 in FIG. 76. If there are no other packets queued, asindicated on a line 631 in FIGS. 75 and 76, the cable modem 12 sends thepacket without a piggyback request to the cable modem termination system10 (see block 633 in FIG. 75) and awaits the arrival of the next packetfrom the external source as indicated at 604. If there are additionalpackets queued as indicated by a line 635 in FIGS. 75 and 76, the cablemodem 12 sends to the cable modem termination system 10 the packetreceived from the external source and piggybacks on this transmittedpacket a request for the next packet in the queue. This is indicated at620 in FIG. 75. The cable modem then returns to processing MAPs at 608looking for additional grants. The cable modem termination system 10then processes the next request from the cable modem.

The cable modem termination system 10 may not grant the full request forbandwidth from the cable modem 12 in the first MAP 170. The cable modemtermination system 10 then provides this partial grant to the cablemodem 12. If the CMTS operates in multiple grant mode, it will place agrant pending or another grant in the MAP in addition to the partialgrant, it sends to the cable modem. The cable modem processes the MAPsas shown in block 608 and sees the grant in line 607. The grant issmaller than the request as on 636 so the cable modem calculates theamount of the packet that will fit in the grant as in block 637. With amultiple grant mode CMTS, the cable modem will see the partial grantwith an additional grant or grant pending in subsequent MAPs as in line607. The cable modem then sends the fragment, without any piggybackrequest as shown in block 628 and line 630 to the cable modemtermination system 10.

The cable modem return to processing map information elements in 608until it gets to the next grant. The cable modem then repeats theprocess of checking to see if the grant is large enough as shown inblock 625.

If the next grant is not large enough, the cable modem repeats theprocess of fragmenting the remaining packet data and, as in 626,checking to see if it needs to send a piggyback request based onadditional grants or grant pendings in the MAP.

If the grant is large enough to transmit the rest of the packet on line627, the cable modem checks to see if there is another packet enqueuedfor this same SID. If so, the cable modem sends the remaining portion ofthe packet with the fragmentation header containing a piggyback requestfor the amount of time slots needed to transmit the next packet in thequeue as shown in block 620. The cable modem then returns to processingthe MAP information elements. If there is not another packet enqueuedfor this SID, then the cable modem sends the remaining portion of thepacket with fragmentation header containing no piggyback request asshown in 633. The cable modem then returns to 604 to await the arrivalof another packet for transmission.

When the cable modem termination system 10 partially grants the requestfrom the cable modem 12 in the first MAP and fails to provide anadditional grant or grant pending to the cable modem 12 in the firstMAP, the cable modem will not detect additional grants or grant pendingsas on line 632. The cable modem 12 then sends to the cable modemtermination system 10 a fragment of the data packet and a piggybackrequest for the remainder as in 634. When the cable modem hastransmitted the fragment with the piggybacked request as shown on line638, the cable modem returns to processing MAP information elements asin 608 while waiting for additional grants. When the cable modemtermination system receives the fragment with the piggybacked request,the cable modem termination system must decide whether to grant the newrequest or send a partial grant based on the new request. This decisionis based on the scheduling algorithms implemented on the cable modemtermination system.

Any time during the request/grant process, the cable modem terminationsystem could fail to receive a request or the cable modem could fail toreceive a grant for a variety of reasons. As a fail safe mechanism, thecable modem termination system places an acknowledgment time, or ACKtime, in the MAPs it transmits. This ACK time reflects the time of thelast request it has processed for the current MAP. The cable modem usesthis ACK time to determine if its request has been lost. The ACK timeris said to have “expired” when the cable modem is waiting for a grantand receives a MAP with an ACK time later in time than when the cablemodem transmitted its request. As the cable modem is looking for grantsat 609, if the ACK time has not expired as on 644, the cable modemreturns to processing the MAPs as in 608. If the ACK timer does expireas on 646, the cable modem checks to see how many times it has retriedsending the request in 648. If the number of retries is above somethreshold, the retries have been exhausted as on 654 and the cable modemtosses any untransmitted portion of the packet at 656 and awaits thearrival of the next packet. If the ACK timer has expired and the numberof retries have not been exhausted as in arrow 650, the cable modem usesa contention request region to transmit another request for the amountof time slots necessary to transmit the untransmitted portion of thepacket as in 652. The cable modem then returns to processing the MAPS.

The operation of the cable modem in transmitting fragmented data isillustrated by the following example considered with FIG. 77.

-   1. (Requesting State)—CM wants to transmit a 1018 byte packet. CM    calculates how much physical layer overhead (POH) is required a nd    requests the appropriate number of minislots. CM makes a request in    a contention region. Go to step 2.-   2. (Waiting for Grant)—CM monitors MAPs for a grant or grant pending    for this SID. If the CM's ACK time expires before the CM receives a    grant or grant pending, the CM retries requesting for the packet    until the retry count is exhausted—then the CM gives up on that    packet. Go to step 3.-   3. (First Fragment)—Prior to giving up in step 2, the CM sees a    grant for this SID that is less than the requested number of    minislots. The CM calculates how much MAC information can be sent in    the granted number of minislots using the of specified burst    profile. In the example in FIG. 77, the first grant can hold 900    bytes after subtracting the POH. Since the fragment overhead (FRAG    HDR, FHCS, and FCRC) is 16 bytes, 884 bytes of the original packet    can be carried in the fragment. The CM creates a fragment composed    of the FRAG HDR, FHCS, 884 bytes of the original packet, and an    FCRC. The CM marks the fragment as first and prepares to send the    fragment. Go to step 4.-   4. (First Fragment, multiple grant mode)—CM looks to see if there    are any other grants of grant pendings enqueued for this SID. If so,    the CM sends the fragment with the piggyback field in the FRAG HDR    set to zero and awaits the time of the subsequent grant to roll    around.—to step 6. If there are not any grants or grant pendings, go    to step 5.-   5. (First Fragment, piggyback mode)—If there are no other grants or    grant pendings for this SID in this MAP, the CM calculates how many    minislots are required to send the remainder of the fragmental    packet, including the fragmentation overhead, and physical layer    overhead, and inserts this amount into the piggyback field of the    FRAG HDR. The CM then sends the fragment and starts its ACK timer    for the piggyback request. In the example in FIG. 19, the CM sends    up a request for enough minislots to hold the POH plus 150 bytes    (1018−884+16). Go to step 6.-   6. (Waiting for Grant). The CM is now waiting for a grant for the    next fragment. If the CM's ACK timer expires while waiting on this    grant, the CM should send up a request for enough minislots to send    the remainder of the fragmented packet, including the fragmentation    overhead, and physical layer overhead. Go to step 7.-   7. (Receives next fragment grant)—Prior to giving up in step 6, the    CM sees another grant for this SID. The CM checks to see if the    grant size is large enough to hold the remainder of the fragmented    packet, including the fragmentation overhead and physical layer    overhead. If so, go to step 10. If not, go to step 8.-   8. (Middle Fragment, multiple grant mode)—Since the remainder of the    packet (plus overhead) will not fit in the grant, the CM calculates    what portion will fit. The CM encapsulates this portion of the    packet as a middle fragment. The CM then looks for any other grants    or grant pendings enqueued for this SID. If either are present, the    CM sends the fragment with the piggyback field in the FRAG HDR set    to zero and awaits the time of the subsequent grant to roll    around.—go to step 6. If there are not any grants or grant pendings,    go to step 9.-   9. (Middle Fragment, piggyback mode). The CM calculates how many    minislots are required to send the remainder of the fragmented    packet, including the fragmentation overhead and physical layer    overhead, and inserts this amount into the piggyback field of the    FRAG HDR. The CM then sends the fragment and starts its ACK timer    for the piggyback request. Go to step 6.

Reference is made to FIGS. 78 and 79 for a description of anotherembodiment of the invention. In this embodiment, there are wirelesstransmission links between homes 14 and HFC network 1010. Each of homes14 is equipped with radio frequency modem (RFM) 2000. A base station2002 is in wireless RF contact with RFM's 2000. The wirelessarchitecture is similar to a cellular phone system. Code divisionmultiple access (CDMA) transmission could be used between RFM's 2000 andbase station 2002. Base station 2002 is connected by a fiber 2004 to aCMTS hub 2006. Hub 2006 is part of HFC network 1010. Otherwise thecomponents in FIGS. 78 and 79 are the same, and bear the same referencenumerals, as those described in connection with FIGS. 1 and 2. Asillustrated in FIG. 78, CMTS hub 2006 can be integrated in the samecable system that also services CM's connected by fiber to hub 22. Thus,upstream and/or downstream channels can be installed in a home withoutphysically laying cable all the way to the home. If desired, thedownstream channel could be fiber because of the large bandwidthrequirement, and the upstream channel could be wireless because there isa smaller bandwidth requirement.

The described functions of cable modems 1046 and RF modems 2000 could becarried out on a single integrated circuit chip as illustrated in FIG.80. In this chip the output of an RF transmitter 3001 feeds the upstreamchannels of HFC network 1010. The downstream channels of HFC network1010 feed the input of an RF receiver 3002. A time division multipleaccess (TDMA) controller 3004 is connected to the input of thetransmitter. The output of receiver 3002 is connected to TDMA controller3004. An ethernet 3006 serves as an interface between TOMA controller3004 and a PC or other binary signal processing device. TDMA controller3004 could be an application specific circuit or a microprocessorprogrammed to perform the described CMTS functions, includingfragmentation. It is understood that the exemplary data packetfragmentation described herein and shown in the drawings represents onlypresently desired embodiments of the invention. Indeed, variousmodifications and additions may be made to such embodiments withoutdeparting from the spirit and scope of the invention. For example,requests to transmit data from cable modems need not be received by thecable modem termination system and the MAP need not be generated by thecable modem termination system, but rather requests may be received byan autonomous device, which operates independently of the cable modemtermination system, and the MAPs may be generated by this or anotherautonomous device. Thus, these and other modifications and additions maybe obvious to those skilled in the art and may be implemented to adaptthe present invention for use in a variety of different applications.The described fragmentation capability can be enabled or disabled in thecable modems on a selective basis. Specifically, when a cable modemtransmits a registration message to the CMTS at the time that the cablemodem enters service, the acknowledging response of the CMTS includes asignal that either enables or disables fragmentation. If fragmentationis enabled, the cable modem and the CMTS operate as described above tofragment data to be transmitted upstream. If fragmentation is disabled,the cable modem only transmits data to the headend if the granted amountof bandwidth is the same as or larger than the bandwidth required totransmit the data. Alternatively, if fragmentation is disabled, the CMTSonly transmits a grant if the requested bandwidth is the same as orsmaller than the bandwidth available for transmission to the headend.

Method and Apparatus for Reducing Noise in a Bidirectional CableTransmission System

In FIG. 81 a bidirectional radio frequency (RF) cable transmissionsystem has a large number of user terminals connected by a cable networkto a headend or a cable modem termination system (CMTS). Cable-modems(CM1, CM2, . . . CMn) are located at the respective user terminals. AnRF transmitter 331 and an RF receiver 335 are located at the headend. RFtransmitter 331 is connected by a number of downstream channels 338 ofthe cable network to cable modems CM1, CM2, . . . CMn. Cable modems CM1,CM2, . . . CMn are connected by upstream channels 339 to RF receiver335. As represented by blocks H1(f), H2(f), . . . Hn(f), the individualupstream channels 339 are impaired by user specific noise associatedwith the respective cable modems such as multi-path reflections and thelike. In addition, as represented by a summing junction 347, upstreamchannels 339 are also impaired by common noise such as, ingress noise,during upstream transmission. The invention reduces the common noisesymbolized by summing junction 347 and the individual noise symbolizedby H1(f), H2(f), . . . Hn(f).

As shown in FIG. 82, RF receiver 335 at the headend has a down converter349 that shifts the RF signal on the selected upstream channel 339 tobaseband. After passing through a matched filter 350, the basebandsignal is coupled to an adaptive notch filter 351, which is described inmore detail below in connection with FIG. 83. Adaptive notch filter 351is coupled by a demodulator 352 to a generalized decision feedbackequalizer (DFE) 353, which is described in more detail below inconnection with FIG. 84. The output of DFE 353 is connected to a slicer354, which determines the quantized value of the signal. The output ofslicer 354 is fed to forward error correction (FEC) circuitry 355. Theoutput of FEC 355 is the transmitted data in binary form.

As illustrated in FIG. 83, adaptive notch filter 351 is a linear monicfilter having a fixed main tap 356 represented by the coefficient b0 anda plurality of successive variable taps 357 represented by coefficientsb1, b2, . . . , bn. Taps 356 and 357 feed a summing junction 358.

As illustrated in FIG. 84, generalized DFE 353 comprises a feed-forwardequalizer 359 and a feedback equalizer 365 that feed a summing junction366. The output of summing junction 366 is coupled to a slicer 367,which determines the quantized value of the signal applied to decisionfeedback equalizer 353. The output of slicer 367 is coupled to the inputof feedback equalizer 365 and to FEC 355 (FIG. 82). As described in moredetail below, generalized OFE 353 operates in a special way. That is,rather than obtaining the coefficents of the feedforward and feedbackparts together, the coefficients of the feed-forward and feedback partsare trained in a sequential manner. Also, the feedforward equalizer cantake a linear equalizer structure where the main tap location can be anytap location.

As illustrated in FIG. 85, each cable modem has a receiver 368 thatprocesses RF signals transmitted on a selected one of downstreamchannels 338 and a transmitter 369 that sends RF signals to the headendon a selected one of upstream channels 339. A transmit equalizer 371 isconnected in series between a modulator 370 inside transmitter 369 andupstream channels 339. As described in more detail below, thecoefficients for equalizer 371 are transmitted to the cable modem on oneof the downstream channels 338 and coupled by downstream receiver 368 totransmit equalizer 371. Transmit equalizer 371 is also a linearequalizer structure corresponding to the feedforward equalizer 359 atthe headend receiver.

FIG. 86 illustrates the TDMA time slots in a selected one of upstreamchannels 339. As shown, there are ranging slots 372, request slots 373,and user data slots 374. There are also idle slots 375, which are uniqueto the invention. The timing and frequency of slots 372-375 aredetermined at the headend, which sends out TDMA control messages on oneof downstream channels 338 assigned to system management to establish aframing structure for the upstream channels. These TDMA control messagesinclude information about the type of slot, i.e., ranging, request,data, or idle and the service identifier (SID) which uniquely identifieseach cable modem. For a more detailed description of the upstreamchannel management function, reference is made to the above section“Cable Modem Termination System Upstream MAC/PHY Interface”.

FIG. 87 illustrates a method for operating the apparatus in FIGS. 82-85so as to cancel common noise such as ingress and to compensate forindividual noise such as multipath noise, that impairs upstream channels339. As depicted by a block 376, an idle slot is created by the headendwhen the system is powered up and thereafter from time to time wheneverit is desired to re-adjust notch filter 351 (FIG. 82). The idle slot ispart of the upstream TDMA framing structure created at the headend andtransmitted to each cable modem on one of downstream channels 338assigned to system management. The idle slot identifies no cable modemSID and contains no upstream signal. It can be created simply byassigning the SID of a unicast (or reservation) slot to a null value.Since the idle slot is created as part of the TDMA framing structure,its time of arrival at the headend is known. In essence, the idle slotis a known time period during which there is no signal on upstreamchannels 339. Any energy received by receiver 335 at the headend duringthis time period represents common noise such as ingress noise. Insteadof using the described idle slots to create quiet periods on theupstream channels for the purpose of sensing and rejecting common noise,the headend MAC could control the cable modems by means of other typesof downstream messaging to create known upstream quiet periods. In anycase, since the headend MAC creates the quiet periods as part of theuptream TDMA framing structure, the MAC can control the timing of theprocess of adjusting notch filter 351 and FBE 365 to coincide with thequiet periods.

In FIG. 88A, this common noise is illustrated as a noise spike 380superimposed on the frequency response 381 of the selected channel. Asdepicted by a block 377, during the idle slot of each of upstreamchannels 339, the coefficients of adaptive notch filter 351, B1, B2, . .. . Bn−1, are adjusted to minimize its output by for example an LMSprocess. The value of main tap B0 is fixed. This tends to cancel thecommon noise as illustrated by a notch 382 in FIG. 88B, but introducessignal distortion into the frequency response. As depicted by a block378, the coefficients of notch filter 377, namely, B1, B2, . . . . Bn−1,are impressed upon FBE 365 without change to compensate for thedistortion introduced by notch filter 351. FIG. 88C illustrates theresulting frequency response at the output of DFE 353 with a narrowsharp notch 383 that rejects the common noise. FIG. 89A represents atypical spread of signal values in a 16-QAM constellation before commonnoise rejection by notch filter 351 and FBE 365. FIG. 89B represents theafter case. As depicted by a block 379, the coefficients of notch filter351 and FBE 365 are frozen until next time that an idle slot is createdby the headend so the described apparatus can take into account changesin the common noise in upstream channel 339. During the initial setup ata particular RF frequency chosen by the down converter 349 and when thesettings of adaptive notch filter 351 are updated, the correspondingcoefficients of notch filter 351, namely, B1, B2, . . . Bn−1, for theselected channel are recovered and impressed upon notch filter 351 andFBE 365. As a result, adaptive notch filter 351 and FBE 365 cancelcommon noise in the selected channel during the data transmissioninterval that follows. It should be noted that this common noisecancellation is accomplished without a training sequence and appliesequally to each cable modem.

As depicted by a block 384, FFE 359 is adjusted during the rangingprocess in preparation for upstream data transmission from each cablemodem. The common noise has been canceled prior to this adjustment. Whenthe headend assigns a ranging slot to a particular cable modem, themodem sends a packet of ranging data, including a training sequence tothe headend. The training sequence is used to derive coefficients forFFE 359. These coefficients represent the frequency shaping required tocompensate for the individual noise of the particular cable modemtransmitting the ranging packet, i.e., the frequency shaping required toprovide a flat frequency response at the output of DFE 353. As depictedby a block 385, the calculated coefficients are transmitted with the SIDof the cable modem on the one of downstream channels 338 assigned tosystem management. These coefficients are applied to transmit equalizer371 at the selected cable modem. It should be noted that since thecommon noise has been canceled prior to the adjustment, the coefficientsapplied to transmit equalizer 371 do not reflect any common noiserejection. Before the data is transmitted by the cable modem over theassigned one of upstream channels 339, FFE 359 is reset so the taps,except for the main tap, are set to zero. As a result, the apparatuscompensates for individual noise by pre-equalization at the transmittingcable modem, while FFE 359 introduces no compensation except simpletracking.

It should be noted that DFE 353 operates sequentially in the practice ofthe invention. First, FBE 365 is set to compensate for the distortionintroduced by notch filter 351 and frozen until it is reset. Then, FFE359 is used to derive the coefficients for transmit equalizer 371, afterwhich it is reset.

In an alternative embodiment, notch filter 351 oversamples the signal atthe output of matched filter 350. Instead of taps spaced apart in timeby the reciprocal of the baud rate of the received signal [T, 2T, 3T, .. . , T(n−1)] the taps of notch filter 351 are spaced apart in time bythe reciprocal of a multiple of the symbol rate, e.g., four (4) times.In this case, only the values of the taps spaced apart in time by thereciprocal of the baud rate [T, 2T, 3T, . . . , T(n−1)] can be adjustedas described above to minimize the output of notch filter 351 and thetaps (both real and imaginary) between those spaced apart by thereciprocal of the baud rate (both real and imaginary) are set to zero.The main tap of notch filter 351 is fixed at real part equals one (1)and imaginary part equals zero. The tap values of FBE 365 (spaced apartin time by the reciprocal of the baud rate) are directly mapped to theadjusted tap values of notch filter 351. The over sampling of notchfilter 351 facilitates acquisition of the burst signals by demodulator352.

Reference is made to FIG. 90A for a diagram of common noise (such asingress) spikes 386 and 387 superimposed on the frequency response ofone of upstream channels 339. The effect of multi-path is shown by a sag388 near the middle of the frequency response. FIG. 90B represents thepre-equalizing effect introduced by transmit equalizer 371 at thetransmitting cable modem. This pre-equalization compensates for sag 388a in FIG. 90A and thus flattens the overall frequency response of thesignal arriving at receiver 335. It also adds notches 386 a and 387 a atthe frequencies where ingress noise is present.

Referring to FIG. 90B, the adaptive notch filter coefficients can beused via FFT processing for dynamic channel allocation with ingresscancellation to measure the inverse of the channel spectrum or to findthe location of the ingress noise. This facilitates in estimating thechannel quality for selecting an appropriate channel and bandwidth, asshown in FIG. 33.

As used in the claims herein, the term “idle slot” is a time periodknown to the headend in which no signal is being transmitted in theupstream channel. Consequently, any energy detected in the channel atthe headend during this time period is common noise introduced into thecable network.

The described embodiments of the invention are only considered to bepreferred and illustrative of the inventive concept; the scope of theinvention is not to be restricted to such embodiments. Various andnumerous other arrangements may be devised by one skilled in the artwithout departing from the spirit and scope of this invention.

1. A cable modem connected to a cable transmission system to communicatewith a cable modem termination system that has a master clock operatingat a system frequency, the cable modem comprising: a frequencycontrollable oscillator having an oscillator frequency representative ofa local frequency; a demodulator for receiving messages representativeof the system frequency; a comparator for generating an error signalrepresentative of the difference between the local frequency representedby the oscillator frequency and the system frequency represented by themessages; a loop filter having an input to which the error signal isapplied and an output that is applied to the oscillator to control theoscillator frequency, the loop filter having initial coefficients thatdefine a first bandwidth; and a controller that adjusts the loop filterto have coefficients that define a second bandwidth that is smaller thanthe first bandwidth when the error signal drops below a threshold level.2. The cable modem of claim 1, wherein the loop filter comprises alinear loop having linear coefficients and an integral loop havingintegral coefficients and the controller adjusts both the linearcoefficients and the integral coefficients when the error signal dropsbelow a threshold level.
 3. The cable modem of claim 2, additionallycomprising a time stamp generating counter connected to the oscillatorto generate time stamps that are representative of the oscillatorfrequency, the messages received by the demodulator are time stamps, andthe error signal generated by the comparator represents the differencebetween the time stamps.
 4. The cable modem of claim 3, wherein thedemodulator receives messages representative of a slot timing offset ofthe cable modem relative to the cable modem termination system and theslot timing offset messages are added to the time stamps generated bythe counter.
 5. The cable modem of claim 4, wherein the resolution ofthe timing offset messages is equal to or finer than the resolution ofthe time stamps generated by the counter.
 6. The cable modem of claim 3,additionally comprising an acquisition counter connected to thedemodulator to count the number of time stamps received by thedemodulator, the controller measures the error signal after theacquisition counter reaches a threshold value, and the controlleradjusts the linear and integral coefficients if the measured errorsignal drops below the threshold level after the acquisition counterreaches the threshold value.
 7. The cable modem of claim 6, wherein thecontroller resets the acquisition counter if the measured error signaldoes not drop below the threshold level after the acquisition counterreaches the threshold value.
 8. The cable modem of claim 7, wherein thecontroller compares the state of the acquisition counter with thethreshold value, advances the state of the acquisition counter if thestate is below the threshold value, measures the error signal if thestate is above the threshold value, and adjusts the coefficients if themeasured error signal drops below the threshold level.
 9. The cablemodem of claim 1, additionally comprising a controller for averaging theinterarrival time between messages and selecting the coefficients from atable of filter coefficients depending on the average interarrival time.10. A cable modem connected to a cable transmission system tocommunicate with a cable modem termination system (CMTS) that has amaster clock operating at a system frequency, the cable modemcomprising: a frequency controllable oscillator synchronized to themaster clock; a counter operated by the oscillator to generate a timestamp message representative of the frequency of the master clock; atiming of offset detector for generating a timing offset messagerepresentative of the transmission time delay between the cable modemand the CMTS; a summer for adding the time stamp message and the timingoffset message to identify time slots on an upstream channel; adownstream receiver for processing MAP messages that specify time slotsassigned to the cable modem on the upstream channel; an upstream dataqueue for storing data to be transmitted upstream; and an upstreamtransmitter for transmitting data in the queue on the upstream channelwhen the time stamp message and the timing offset message identify timeslots that match the time slots specified by the MAP messages.
 11. Thecable modem of claim 10, wherein the downstream receiver recovers timestamp messages sent downstream by the CMTS responsive to the masterclock, and the cable modem additionally comprises a control loop thatadjusts the frequency of the oscillator responsive to the differencebetween the time stamp messages sent downstream by the CMTS and timestamp messages generated by the counter to synchronize the oscillator tothe master clock.
 12. The cable modem of claim 10, wherein thedownstream receiver recovers a ranging signal sent downstream by theCMTS and the timing offset detector generates the timing offset messagefrom the ranging signal.
 13. A cable modem termination system (CMTS)comprising: a time stamp generator that generates time stamp messagesfor synchronizing cable modems to each other; a downstream data queuethat has frame boundaries between portions of the data in the queue; adownstream processor that formats data from the queue into a datastream; a time stamp send generator for issuing time stamp sendcommands; and a downstream transmitter connected to the downstreamprocessor to send the formatted data, connected to the time stampgenerator to receive the time stamp messages, and connected to the timestamp send generator to insert time stamp messages from the time stampgenerator into the data stream responsive to the time stamp sendcommands.
 14. The CMTS of claim 13, wherein the time stamp generatorcomprises a master clock and a counter operated by the master clock togenerate time stamp messages that represent the current state of thecounter, the downstream processor being connected to the counter toinsert time stamp messages from the counter into the data streamresponsive to the time stamp send commands.
 15. The CMTS of claim 14,wherein the time stamp send generator comprise a counter operated by themaster clock.
 16. The CMTS of claim 15, wherein the counter isconfigured to generate a time stamp send command after the counterreaches a predetermined count.
 17. The CMTS of claim 16, wherein thepredetermined count is programmable.
 18. A method for coordinating thetransmission of data to a cable headend on an upstream channel of acable system from one of a plurality of cable subscriber stationslocated different distances from the headend, the method comprising thesteps of: synchronizing a frequency controllable local clock at thesubscriber station to a master clock at the headend; generating a timestamp message from the local clock representative of the frequency ofthe master clock; receiving at the subscriber station a timingcorrection message generated at the headend, the timing correctionmessage representing the transmission time delay between the onesubscriber station and the headend relative to the other subscriberstations; adding the time stamp message and the timing correctionmessage to define time slots for upstream transmissions from thesubscriber stations; and transmitting data from the one subscriberstation to the headend in the time slots.
 19. The method of claim 18,wherein the timing correction message is referenced to the master clock.20. The method of claim 18, wherein the local clock is an oscillator,the step of generating a time stamp message at the subscriber stationcomprises counting cycles of the oscillator, and the step ofsynchronizing the local clock to the master clock comprises generatingan error signal representative of the difference between the time stampmessage generated at the subscriber station and a time stamp messagegenerated at the headend by the master clock and controlling thefrequency of the local clock to reduce the difference.
 21. The method ofclaim 20, wherein the step of receiving at the subscriber station atiming correction message comprises: sending a command from the headendto the subscriber station to transmit a timing offset signal at aprescribed time referenced to the subscriber's local clock, there beingfor each subscriber station an expected time of arrival at the headendbased on the distance from the headend; transmitting the timing offsetsignal from the subscriber station at the prescribed time referenced tothe subscriber's local clock; receiving the timing offset signal fromthe subscriber station at the headend; generating at the headend for thesubscriber station a timing correction message, the timing correctionmessage representing the difference between the actual time of arrivalof the timing offset signal at the headend and the expected time ofarrival referenced to the master clock; and transmitting the timingcorrection message for receipt by the subscriber station.
 22. In a cabletransmission system having a headend, a plurality of cable modemslocated different distances from the headend, and a cable network thatconnects the headend to the cable modems, a combination comprising: amaster clock at the headend; a local clock at each cable modem; meansfor synchronizing each local clock to the master clock; means forsending from the headend to each cable modem a command to transmit atiming offset signal at a prescribed time referenced to the local clock,there being for each cable modem an expected time of arrival at theheadend based on the distance from the headend; means for transmittingthe timing offset signal from each cable modem at the prescribed timereferenced to the local clock; means for receiving the timing offsetsignal from each cable modem at the headend; means for generating at theheadend for each cable modem a timing correction message, the timingcorrection message representing the difference between the actual timeof arrival of the timing offset signal at the headend and the expectedtime of arrival referenced to the headend clock; means for transmittingthe timing correction messages to the respective cable modems; meansresponsive to the local clock at each cable modem for generating acontinuously incremented local clock timing signal; means at each cablemodem for adding the timing correction message to the local clock timingsignal to produce a time slot defining signal corrected for theparticular cable modem; means for transmitting data from each cablemodem to the headend in time slots allocated from the headend responsiveto the time slot defining signal so as to place the data from the cablemodems in the allocated time slots.
 23. The combination of claim 22,additionally comprising means responsive to the master clock forgenerating at the headend a continuously incremented master clock timingsignal, means for generating MAP messages to allocate time slots to thecable modems based on the master clock timing signal and means fortransmitting the MAP messages to the cable modems.
 24. The combinationof claim 23, wherein the synchronizing means comprises means fortransmitting the master clock timing signal to each cable modem, meansfor comparing the master clock timing signal and the local clock timingsignal to generate an error signal, and means for correcting thefrequency of the local clock responsive to the error signal.